| Message ID | 20210108001043.12683-1-sargun@sargun.me |
|---|---|
| State | Accepted |
| Commit | 335d3fc57941e5c6164c69d439aec1cb7a800876 |
| Headers | show |
| Series | [v4] overlay: Implement volatile-specific fsync error behaviour | expand |
On Thu, Jan 07, 2021 at 04:10:43PM -0800, Sargun Dhillon wrote: > Overlayfs's volatile option allows the user to bypass all forced sync calls > to the upperdir filesystem. This comes at the cost of safety. We can never > ensure that the user's data is intact, but we can make a best effort to > expose whether or not the data is likely to be in a bad state. > > The best way to handle this in the time being is that if an overlayfs's > upperdir experiences an error after a volatile mount occurs, that error > will be returned on fsync, fdatasync, sync, and syncfs. This is > contradictory to the traditional behaviour of VFS which fails the call > once, and only raises an error if a subsequent fsync error has occurred, > and been raised by the filesystem. > > One awkward aspect of the patch is that we have to manually set the > superblock's errseq_t after the sync_fs callback as opposed to just > returning an error from syncfs. This is because the call chain looks > something like this: > > sys_syncfs -> > sync_filesystem -> > __sync_filesystem -> > /* The return value is ignored here > sb->s_op->sync_fs(sb) > _sync_blockdev > /* Where the VFS fetches the error to raise to userspace */ > errseq_check_and_advance > > Because of this we call errseq_set every time the sync_fs callback occurs. > Due to the nature of this seen / unseen dichotomy, if the upperdir is an > inconsistent state at the initial mount time, overlayfs will refuse to > mount, as overlayfs cannot get a snapshot of the upperdir's errseq that > will increment on error until the user calls syncfs. > > Signed-off-by: Sargun Dhillon <sargun@sargun.me> > Suggested-by: Amir Goldstein <amir73il@gmail.com> > Reviewed-by: Amir Goldstein <amir73il@gmail.com> > Fixes: c86243b090bc ("ovl: provide a mount option "volatile"") > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-unionfs@vger.kernel.org > Cc: stable@vger.kernel.org > Cc: Jeff Layton <jlayton@redhat.com> > Cc: Miklos Szeredi <miklos@szeredi.hu> > Cc: Amir Goldstein <amir73il@gmail.com> > Cc: Vivek Goyal <vgoyal@redhat.com> > Cc: Matthew Wilcox <willy@infradead.org> Looks good to me. Reviewed-by: Vivek Goyal <vgoyal@redhat.com> Thanks Vivek > --- > Documentation/filesystems/overlayfs.rst | 8 ++++++ > fs/overlayfs/file.c | 5 ++-- > fs/overlayfs/overlayfs.h | 1 + > fs/overlayfs/ovl_entry.h | 2 ++ > fs/overlayfs/readdir.c | 5 ++-- > fs/overlayfs/super.c | 34 ++++++++++++++++++++----- > fs/overlayfs/util.c | 27 ++++++++++++++++++++ > 7 files changed, 71 insertions(+), 11 deletions(-) > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > index 580ab9a0fe31..137afeb3f581 100644 > --- a/Documentation/filesystems/overlayfs.rst > +++ b/Documentation/filesystems/overlayfs.rst > @@ -575,6 +575,14 @@ without significant effort. > The advantage of mounting with the "volatile" option is that all forms of > sync calls to the upper filesystem are omitted. > > +In order to avoid a giving a false sense of safety, the syncfs (and fsync) > +semantics of volatile mounts are slightly different than that of the rest of > +VFS. If any writeback error occurs on the upperdir's filesystem after a > +volatile mount takes place, all sync functions will return an error. Once this > +condition is reached, the filesystem will not recover, and every subsequent sync > +call will return an error, even if the upperdir has not experience a new error > +since the last sync call. > + > When overlay is mounted with "volatile" option, the directory > "$workdir/work/incompat/volatile" is created. During next mount, overlay > checks for this directory and refuses to mount if present. This is a strong > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c > index a1f72ac053e5..5c5c3972ebd0 100644 > --- a/fs/overlayfs/file.c > +++ b/fs/overlayfs/file.c > @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) > const struct cred *old_cred; > int ret; > > - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb))) > - return 0; > + ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb)); > + if (ret <= 0) > + return ret; > > ret = ovl_real_fdget_meta(file, &real, !datasync); > if (ret) > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h > index f8880aa2ba0e..9f7af98ae200 100644 > --- a/fs/overlayfs/overlayfs.h > +++ b/fs/overlayfs/overlayfs.h > @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry); > bool ovl_is_metacopy_dentry(struct dentry *dentry); > char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > int padding); > +int ovl_sync_status(struct ovl_fs *ofs); > > static inline bool ovl_is_impuredir(struct super_block *sb, > struct dentry *dentry) > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h > index 1b5a2094df8e..b208eba5d0b6 100644 > --- a/fs/overlayfs/ovl_entry.h > +++ b/fs/overlayfs/ovl_entry.h > @@ -79,6 +79,8 @@ struct ovl_fs { > atomic_long_t last_ino; > /* Whiteout dentry cache */ > struct dentry *whiteout; > + /* r/o snapshot of upperdir sb's only taken on volatile mounts */ > + errseq_t errseq; > }; > > static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c > index 01620ebae1bd..a273ef901e57 100644 > --- a/fs/overlayfs/readdir.c > +++ b/fs/overlayfs/readdir.c > @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end, > struct file *realfile; > int err; > > - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb))) > - return 0; > + err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb)); > + if (err <= 0) > + return err; > > realfile = ovl_dir_real_file(file, true); > err = PTR_ERR_OR_ZERO(realfile); > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > index 290983bcfbb3..d23177a53c95 100644 > --- a/fs/overlayfs/super.c > +++ b/fs/overlayfs/super.c > @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait) > struct super_block *upper_sb; > int ret; > > - if (!ovl_upper_mnt(ofs)) > - return 0; > + ret = ovl_sync_status(ofs); > + /* > + * We have to always set the err, because the return value isn't > + * checked in syncfs, and instead indirectly return an error via > + * the sb's writeback errseq, which VFS inspects after this call. > + */ > + if (ret < 0) { > + errseq_set(&sb->s_wb_err, -EIO); > + return -EIO; > + } > + > + if (!ret) > + return ret; > > - if (!ovl_should_sync(ofs)) > - return 0; > /* > * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC). > * All the super blocks will be iterated, including upper_sb. > @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > sb->s_op = &ovl_super_operations; > > if (ofs->config.upperdir) { > + struct super_block *upper_sb; > + > if (!ofs->config.workdir) { > pr_err("missing 'workdir'\n"); > goto out_err; > @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > if (err) > goto out_err; > > + upper_sb = ovl_upper_mnt(ofs)->mnt_sb; > + if (!ovl_should_sync(ofs)) { > + ofs->errseq = errseq_sample(&upper_sb->s_wb_err); > + if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) { > + err = -EIO; > + pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n"); > + goto out_err; > + } > + } > + > err = ovl_get_workdir(sb, ofs, &upperpath); > if (err) > goto out_err; > @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > if (!ofs->workdir) > sb->s_flags |= SB_RDONLY; > > - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth; > - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran; > - > + sb->s_stack_depth = upper_sb->s_stack_depth; > + sb->s_time_gran = upper_sb->s_time_gran; > } > oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers); > err = PTR_ERR(oe); > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c > index 23f475627d07..6e7b8c882045 100644 > --- a/fs/overlayfs/util.c > +++ b/fs/overlayfs/util.c > @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > kfree(buf); > return ERR_PTR(res); > } > + > +/* > + * ovl_sync_status() - Check fs sync status for volatile mounts > + * > + * Returns 1 if this is not a volatile mount and a real sync is required. > + * > + * Returns 0 if syncing can be skipped because mount is volatile, and no errors > + * have occurred on the upperdir since the mount. > + * > + * Returns -errno if it is a volatile mount, and the error that occurred since > + * the last mount. If the error code changes, it'll return the latest error > + * code. > + */ > + > +int ovl_sync_status(struct ovl_fs *ofs) > +{ > + struct vfsmount *mnt; > + > + if (ovl_should_sync(ofs)) > + return 1; > + > + mnt = ovl_upper_mnt(ofs); > + if (!mnt) > + return 0; > + > + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq); > +} > -- > 2.25.1 >
On Thu, 2021-01-07 at 16:10 -0800, Sargun Dhillon wrote: > Overlayfs's volatile option allows the user to bypass all forced sync calls > to the upperdir filesystem. This comes at the cost of safety. We can never > ensure that the user's data is intact, but we can make a best effort to > expose whether or not the data is likely to be in a bad state. > > The best way to handle this in the time being is that if an overlayfs's > upperdir experiences an error after a volatile mount occurs, that error > will be returned on fsync, fdatasync, sync, and syncfs. This is > contradictory to the traditional behaviour of VFS which fails the call > once, and only raises an error if a subsequent fsync error has occurred, > and been raised by the filesystem. > > One awkward aspect of the patch is that we have to manually set the > superblock's errseq_t after the sync_fs callback as opposed to just > returning an error from syncfs. This is because the call chain looks > something like this: > > sys_syncfs -> > sync_filesystem -> > __sync_filesystem -> > /* The return value is ignored here > sb->s_op->sync_fs(sb) > _sync_blockdev > /* Where the VFS fetches the error to raise to userspace */ > errseq_check_and_advance > > Because of this we call errseq_set every time the sync_fs callback occurs. > Due to the nature of this seen / unseen dichotomy, if the upperdir is an > inconsistent state at the initial mount time, overlayfs will refuse to > mount, as overlayfs cannot get a snapshot of the upperdir's errseq that > will increment on error until the user calls syncfs. > > Signed-off-by: Sargun Dhillon <sargun@sargun.me> > Suggested-by: Amir Goldstein <amir73il@gmail.com> > Reviewed-by: Amir Goldstein <amir73il@gmail.com> > Fixes: c86243b090bc ("ovl: provide a mount option "volatile"") > Cc: linux-fsdevel@vger.kernel.org > Cc: linux-unionfs@vger.kernel.org > Cc: stable@vger.kernel.org > Cc: Jeff Layton <jlayton@redhat.com> > Cc: Miklos Szeredi <miklos@szeredi.hu> > Cc: Amir Goldstein <amir73il@gmail.com> > Cc: Vivek Goyal <vgoyal@redhat.com> > Cc: Matthew Wilcox <willy@infradead.org> > --- > Documentation/filesystems/overlayfs.rst | 8 ++++++ > fs/overlayfs/file.c | 5 ++-- > fs/overlayfs/overlayfs.h | 1 + > fs/overlayfs/ovl_entry.h | 2 ++ > fs/overlayfs/readdir.c | 5 ++-- > fs/overlayfs/super.c | 34 ++++++++++++++++++++----- > fs/overlayfs/util.c | 27 ++++++++++++++++++++ > 7 files changed, 71 insertions(+), 11 deletions(-) > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > index 580ab9a0fe31..137afeb3f581 100644 > --- a/Documentation/filesystems/overlayfs.rst > +++ b/Documentation/filesystems/overlayfs.rst > @@ -575,6 +575,14 @@ without significant effort. > The advantage of mounting with the "volatile" option is that all forms of > sync calls to the upper filesystem are omitted. > > > > > +In order to avoid a giving a false sense of safety, the syncfs (and fsync) > +semantics of volatile mounts are slightly different than that of the rest of > +VFS. If any writeback error occurs on the upperdir's filesystem after a > +volatile mount takes place, all sync functions will return an error. Once this > +condition is reached, the filesystem will not recover, and every subsequent sync > +call will return an error, even if the upperdir has not experience a new error > +since the last sync call. > + > When overlay is mounted with "volatile" option, the directory > "$workdir/work/incompat/volatile" is created. During next mount, overlay > checks for this directory and refuses to mount if present. This is a strong > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c > index a1f72ac053e5..5c5c3972ebd0 100644 > --- a/fs/overlayfs/file.c > +++ b/fs/overlayfs/file.c > @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) > const struct cred *old_cred; > int ret; > > > > > - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb))) > - return 0; > + ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb)); > + if (ret <= 0) > + return ret; > > > > > ret = ovl_real_fdget_meta(file, &real, !datasync); > if (ret) > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h > index f8880aa2ba0e..9f7af98ae200 100644 > --- a/fs/overlayfs/overlayfs.h > +++ b/fs/overlayfs/overlayfs.h > @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry); > bool ovl_is_metacopy_dentry(struct dentry *dentry); > char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > int padding); > +int ovl_sync_status(struct ovl_fs *ofs); > > > > > static inline bool ovl_is_impuredir(struct super_block *sb, > struct dentry *dentry) > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h > index 1b5a2094df8e..b208eba5d0b6 100644 > --- a/fs/overlayfs/ovl_entry.h > +++ b/fs/overlayfs/ovl_entry.h > @@ -79,6 +79,8 @@ struct ovl_fs { > atomic_long_t last_ino; > /* Whiteout dentry cache */ > struct dentry *whiteout; > + /* r/o snapshot of upperdir sb's only taken on volatile mounts */ > + errseq_t errseq; > }; > > > > > static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c > index 01620ebae1bd..a273ef901e57 100644 > --- a/fs/overlayfs/readdir.c > +++ b/fs/overlayfs/readdir.c > @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end, > struct file *realfile; > int err; > > > > > - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb))) > - return 0; > + err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb)); > + if (err <= 0) > + return err; > > > > > realfile = ovl_dir_real_file(file, true); > err = PTR_ERR_OR_ZERO(realfile); > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > index 290983bcfbb3..d23177a53c95 100644 > --- a/fs/overlayfs/super.c > +++ b/fs/overlayfs/super.c > @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait) > struct super_block *upper_sb; > int ret; > > > > > - if (!ovl_upper_mnt(ofs)) > - return 0; > + ret = ovl_sync_status(ofs); > + /* > + * We have to always set the err, because the return value isn't > + * checked in syncfs, and instead indirectly return an error via > + * the sb's writeback errseq, which VFS inspects after this call. > + */ > + if (ret < 0) { > + errseq_set(&sb->s_wb_err, -EIO); > + return -EIO; > + } > + Apologies for the late review: You're converting the error you got back from errseq_check to -EIO unconditionally here. Why? Would it not be better to just pass the error through? Like this: if (ret < 0) { errseq_set(&sb->s_wb_err, ret); return ret; } ? > + if (!ret) > + return ret; > > > > > - if (!ovl_should_sync(ofs)) > - return 0; > /* > * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC). > * All the super blocks will be iterated, including upper_sb. > @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > sb->s_op = &ovl_super_operations; > > > > > if (ofs->config.upperdir) { > + struct super_block *upper_sb; > + > if (!ofs->config.workdir) { > pr_err("missing 'workdir'\n"); > goto out_err; > @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > if (err) > goto out_err; > > > > > + upper_sb = ovl_upper_mnt(ofs)->mnt_sb; > + if (!ovl_should_sync(ofs)) { > + ofs->errseq = errseq_sample(&upper_sb->s_wb_err); > + if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) { > + err = -EIO; > + pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n"); > + goto out_err; > + } > + } > + > err = ovl_get_workdir(sb, ofs, &upperpath); > if (err) > goto out_err; > @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > if (!ofs->workdir) > sb->s_flags |= SB_RDONLY; > > > > > - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth; > - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran; > - > + sb->s_stack_depth = upper_sb->s_stack_depth; > + sb->s_time_gran = upper_sb->s_time_gran; > } > oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers); > err = PTR_ERR(oe); > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c > index 23f475627d07..6e7b8c882045 100644 > --- a/fs/overlayfs/util.c > +++ b/fs/overlayfs/util.c > @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > kfree(buf); > return ERR_PTR(res); > } > + > +/* > + * ovl_sync_status() - Check fs sync status for volatile mounts > + * > + * Returns 1 if this is not a volatile mount and a real sync is required. > + * > + * Returns 0 if syncing can be skipped because mount is volatile, and no errors > + * have occurred on the upperdir since the mount. > + * > + * Returns -errno if it is a volatile mount, and the error that occurred since > + * the last mount. If the error code changes, it'll return the latest error > + * code. > + */ > + > +int ovl_sync_status(struct ovl_fs *ofs) > +{ > + struct vfsmount *mnt; > + > + if (ovl_should_sync(ofs)) > + return 1; > + > + mnt = ovl_upper_mnt(ofs); > + if (!mnt) > + return 0; > + > + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq); > +} Aside from my minor question about converting the error unconditionally, this all looks fine. You can add: Reviewed-by: Jeff Layton <jlayton@kernel.org> Nice work!
On Tue, Jan 19, 2021 at 06:49:36AM -0500, Jeff Layton wrote: > On Thu, 2021-01-07 at 16:10 -0800, Sargun Dhillon wrote: > > Overlayfs's volatile option allows the user to bypass all forced sync calls > > to the upperdir filesystem. This comes at the cost of safety. We can never > > ensure that the user's data is intact, but we can make a best effort to > > expose whether or not the data is likely to be in a bad state. > > > > The best way to handle this in the time being is that if an overlayfs's > > upperdir experiences an error after a volatile mount occurs, that error > > will be returned on fsync, fdatasync, sync, and syncfs. This is > > contradictory to the traditional behaviour of VFS which fails the call > > once, and only raises an error if a subsequent fsync error has occurred, > > and been raised by the filesystem. > > > > One awkward aspect of the patch is that we have to manually set the > > superblock's errseq_t after the sync_fs callback as opposed to just > > returning an error from syncfs. This is because the call chain looks > > something like this: > > > > sys_syncfs -> > > sync_filesystem -> > > __sync_filesystem -> > > /* The return value is ignored here > > sb->s_op->sync_fs(sb) > > _sync_blockdev > > /* Where the VFS fetches the error to raise to userspace */ > > errseq_check_and_advance > > > > Because of this we call errseq_set every time the sync_fs callback occurs. > > Due to the nature of this seen / unseen dichotomy, if the upperdir is an > > inconsistent state at the initial mount time, overlayfs will refuse to > > mount, as overlayfs cannot get a snapshot of the upperdir's errseq that > > will increment on error until the user calls syncfs. > > > > Signed-off-by: Sargun Dhillon <sargun@sargun.me> > > Suggested-by: Amir Goldstein <amir73il@gmail.com> > > Reviewed-by: Amir Goldstein <amir73il@gmail.com> > > Fixes: c86243b090bc ("ovl: provide a mount option "volatile"") > > Cc: linux-fsdevel@vger.kernel.org > > Cc: linux-unionfs@vger.kernel.org > > Cc: stable@vger.kernel.org > > Cc: Jeff Layton <jlayton@redhat.com> > > Cc: Miklos Szeredi <miklos@szeredi.hu> > > Cc: Amir Goldstein <amir73il@gmail.com> > > Cc: Vivek Goyal <vgoyal@redhat.com> > > Cc: Matthew Wilcox <willy@infradead.org> > > --- > > Documentation/filesystems/overlayfs.rst | 8 ++++++ > > fs/overlayfs/file.c | 5 ++-- > > fs/overlayfs/overlayfs.h | 1 + > > fs/overlayfs/ovl_entry.h | 2 ++ > > fs/overlayfs/readdir.c | 5 ++-- > > fs/overlayfs/super.c | 34 ++++++++++++++++++++----- > > fs/overlayfs/util.c | 27 ++++++++++++++++++++ > > 7 files changed, 71 insertions(+), 11 deletions(-) > > > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > > index 580ab9a0fe31..137afeb3f581 100644 > > --- a/Documentation/filesystems/overlayfs.rst > > +++ b/Documentation/filesystems/overlayfs.rst > > @@ -575,6 +575,14 @@ without significant effort. > > The advantage of mounting with the "volatile" option is that all forms of > > sync calls to the upper filesystem are omitted. > > > > > > > > > > +In order to avoid a giving a false sense of safety, the syncfs (and fsync) > > +semantics of volatile mounts are slightly different than that of the rest of > > +VFS. If any writeback error occurs on the upperdir's filesystem after a > > +volatile mount takes place, all sync functions will return an error. Once this > > +condition is reached, the filesystem will not recover, and every subsequent sync > > +call will return an error, even if the upperdir has not experience a new error > > +since the last sync call. > > + > > When overlay is mounted with "volatile" option, the directory > > "$workdir/work/incompat/volatile" is created. During next mount, overlay > > checks for this directory and refuses to mount if present. This is a strong > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c > > index a1f72ac053e5..5c5c3972ebd0 100644 > > --- a/fs/overlayfs/file.c > > +++ b/fs/overlayfs/file.c > > @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) > > const struct cred *old_cred; > > int ret; > > > > > > > > > > - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb))) > > - return 0; > > + ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb)); > > + if (ret <= 0) > > + return ret; > > > > > > > > > > ret = ovl_real_fdget_meta(file, &real, !datasync); > > if (ret) > > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h > > index f8880aa2ba0e..9f7af98ae200 100644 > > --- a/fs/overlayfs/overlayfs.h > > +++ b/fs/overlayfs/overlayfs.h > > @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry); > > bool ovl_is_metacopy_dentry(struct dentry *dentry); > > char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > > int padding); > > +int ovl_sync_status(struct ovl_fs *ofs); > > > > > > > > > > static inline bool ovl_is_impuredir(struct super_block *sb, > > struct dentry *dentry) > > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h > > index 1b5a2094df8e..b208eba5d0b6 100644 > > --- a/fs/overlayfs/ovl_entry.h > > +++ b/fs/overlayfs/ovl_entry.h > > @@ -79,6 +79,8 @@ struct ovl_fs { > > atomic_long_t last_ino; > > /* Whiteout dentry cache */ > > struct dentry *whiteout; > > + /* r/o snapshot of upperdir sb's only taken on volatile mounts */ > > + errseq_t errseq; > > }; > > > > > > > > > > static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c > > index 01620ebae1bd..a273ef901e57 100644 > > --- a/fs/overlayfs/readdir.c > > +++ b/fs/overlayfs/readdir.c > > @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end, > > struct file *realfile; > > int err; > > > > > > > > > > - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb))) > > - return 0; > > + err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb)); > > + if (err <= 0) > > + return err; > > > > > > > > > > realfile = ovl_dir_real_file(file, true); > > err = PTR_ERR_OR_ZERO(realfile); > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > > index 290983bcfbb3..d23177a53c95 100644 > > --- a/fs/overlayfs/super.c > > +++ b/fs/overlayfs/super.c > > @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait) > > struct super_block *upper_sb; > > int ret; > > > > > > > > > > - if (!ovl_upper_mnt(ofs)) > > - return 0; > > + ret = ovl_sync_status(ofs); > > + /* > > + * We have to always set the err, because the return value isn't > > + * checked in syncfs, and instead indirectly return an error via > > + * the sb's writeback errseq, which VFS inspects after this call. > > + */ > > + if (ret < 0) { > > + errseq_set(&sb->s_wb_err, -EIO); > > + return -EIO; > > + } > > + > > Apologies for the late review: > > You're converting the error you got back from errseq_check to -EIO > unconditionally here. Why? Would it not be better to just pass the error > through? Like this: > I believe Vivek brought this up as a potentially confusing issue. For example, you can end up with an ENOSPC on a write, and then delete a file, and still get ENOSPC. It's just easier to return EIO, given there's not really anything the user can do to recover the system. > if (ret < 0) { > errseq_set(&sb->s_wb_err, ret); > return ret; > } > > ? > > > + if (!ret) > > + return ret; > > > > > > > > > > - if (!ovl_should_sync(ofs)) > > - return 0; > > /* > > * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC). > > * All the super blocks will be iterated, including upper_sb. > > @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > > sb->s_op = &ovl_super_operations; > > > > > > > > > > if (ofs->config.upperdir) { > > + struct super_block *upper_sb; > > + > > if (!ofs->config.workdir) { > > pr_err("missing 'workdir'\n"); > > goto out_err; > > @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > > if (err) > > goto out_err; > > > > > > > > > > + upper_sb = ovl_upper_mnt(ofs)->mnt_sb; > > + if (!ovl_should_sync(ofs)) { > > + ofs->errseq = errseq_sample(&upper_sb->s_wb_err); > > + if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) { > > + err = -EIO; > > + pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n"); > > + goto out_err; > > + } > > + } > > + > > err = ovl_get_workdir(sb, ofs, &upperpath); > > if (err) > > goto out_err; > > @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > > if (!ofs->workdir) > > sb->s_flags |= SB_RDONLY; > > > > > > > > > > - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth; > > - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran; > > - > > + sb->s_stack_depth = upper_sb->s_stack_depth; > > + sb->s_time_gran = upper_sb->s_time_gran; > > } > > oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers); > > err = PTR_ERR(oe); > > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c > > index 23f475627d07..6e7b8c882045 100644 > > --- a/fs/overlayfs/util.c > > +++ b/fs/overlayfs/util.c > > @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > > kfree(buf); > > return ERR_PTR(res); > > } > > + > > +/* > > + * ovl_sync_status() - Check fs sync status for volatile mounts > > + * > > + * Returns 1 if this is not a volatile mount and a real sync is required. > > + * > > + * Returns 0 if syncing can be skipped because mount is volatile, and no errors > > + * have occurred on the upperdir since the mount. > > + * > > + * Returns -errno if it is a volatile mount, and the error that occurred since > > + * the last mount. If the error code changes, it'll return the latest error > > + * code. > > + */ > > + > > +int ovl_sync_status(struct ovl_fs *ofs) > > +{ > > + struct vfsmount *mnt; > > + > > + if (ovl_should_sync(ofs)) > > + return 1; > > + > > + mnt = ovl_upper_mnt(ofs); > > + if (!mnt) > > + return 0; > > + > > + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq); > > +} > > Aside from my minor question about converting the error unconditionally, > this all looks fine. You can add: > > Reviewed-by: Jeff Layton <jlayton@kernel.org> > > Nice work! > Thanks!
On Fri, Jan 8, 2021 at 1:10 AM Sargun Dhillon <sargun@sargun.me> wrote: > > Overlayfs's volatile option allows the user to bypass all forced sync calls > to the upperdir filesystem. This comes at the cost of safety. We can never > ensure that the user's data is intact, but we can make a best effort to > expose whether or not the data is likely to be in a bad state. > > The best way to handle this in the time being is that if an overlayfs's > upperdir experiences an error after a volatile mount occurs, that error > will be returned on fsync, fdatasync, sync, and syncfs. This is > contradictory to the traditional behaviour of VFS which fails the call > once, and only raises an error if a subsequent fsync error has occurred, > and been raised by the filesystem. > > One awkward aspect of the patch is that we have to manually set the > superblock's errseq_t after the sync_fs callback as opposed to just > returning an error from syncfs. This is because the call chain looks > something like this: > > sys_syncfs -> > sync_filesystem -> > __sync_filesystem -> > /* The return value is ignored here > sb->s_op->sync_fs(sb) > _sync_blockdev > /* Where the VFS fetches the error to raise to userspace */ > errseq_check_and_advance > > Because of this we call errseq_set every time the sync_fs callback occurs. > Due to the nature of this seen / unseen dichotomy, if the upperdir is an > inconsistent state at the initial mount time, overlayfs will refuse to > mount, as overlayfs cannot get a snapshot of the upperdir's errseq that > will increment on error until the user calls syncfs. Thanks, this makes sense. Queued for v4.11. Miklos
diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst index 580ab9a0fe31..137afeb3f581 100644 --- a/Documentation/filesystems/overlayfs.rst +++ b/Documentation/filesystems/overlayfs.rst @@ -575,6 +575,14 @@ without significant effort. The advantage of mounting with the "volatile" option is that all forms of sync calls to the upper filesystem are omitted. +In order to avoid a giving a false sense of safety, the syncfs (and fsync) +semantics of volatile mounts are slightly different than that of the rest of +VFS. If any writeback error occurs on the upperdir's filesystem after a +volatile mount takes place, all sync functions will return an error. Once this +condition is reached, the filesystem will not recover, and every subsequent sync +call will return an error, even if the upperdir has not experience a new error +since the last sync call. + When overlay is mounted with "volatile" option, the directory "$workdir/work/incompat/volatile" is created. During next mount, overlay checks for this directory and refuses to mount if present. This is a strong diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c index a1f72ac053e5..5c5c3972ebd0 100644 --- a/fs/overlayfs/file.c +++ b/fs/overlayfs/file.c @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) const struct cred *old_cred; int ret; - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb))) - return 0; + ret = ovl_sync_status(OVL_FS(file_inode(file)->i_sb)); + if (ret <= 0) + return ret; ret = ovl_real_fdget_meta(file, &real, !datasync); if (ret) diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h index f8880aa2ba0e..9f7af98ae200 100644 --- a/fs/overlayfs/overlayfs.h +++ b/fs/overlayfs/overlayfs.h @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry); bool ovl_is_metacopy_dentry(struct dentry *dentry); char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, int padding); +int ovl_sync_status(struct ovl_fs *ofs); static inline bool ovl_is_impuredir(struct super_block *sb, struct dentry *dentry) diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h index 1b5a2094df8e..b208eba5d0b6 100644 --- a/fs/overlayfs/ovl_entry.h +++ b/fs/overlayfs/ovl_entry.h @@ -79,6 +79,8 @@ struct ovl_fs { atomic_long_t last_ino; /* Whiteout dentry cache */ struct dentry *whiteout; + /* r/o snapshot of upperdir sb's only taken on volatile mounts */ + errseq_t errseq; }; static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c index 01620ebae1bd..a273ef901e57 100644 --- a/fs/overlayfs/readdir.c +++ b/fs/overlayfs/readdir.c @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end, struct file *realfile; int err; - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb))) - return 0; + err = ovl_sync_status(OVL_FS(file->f_path.dentry->d_sb)); + if (err <= 0) + return err; realfile = ovl_dir_real_file(file, true); err = PTR_ERR_OR_ZERO(realfile); diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c index 290983bcfbb3..d23177a53c95 100644 --- a/fs/overlayfs/super.c +++ b/fs/overlayfs/super.c @@ -261,11 +261,20 @@ static int ovl_sync_fs(struct super_block *sb, int wait) struct super_block *upper_sb; int ret; - if (!ovl_upper_mnt(ofs)) - return 0; + ret = ovl_sync_status(ofs); + /* + * We have to always set the err, because the return value isn't + * checked in syncfs, and instead indirectly return an error via + * the sb's writeback errseq, which VFS inspects after this call. + */ + if (ret < 0) { + errseq_set(&sb->s_wb_err, -EIO); + return -EIO; + } + + if (!ret) + return ret; - if (!ovl_should_sync(ofs)) - return 0; /* * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC). * All the super blocks will be iterated, including upper_sb. @@ -1927,6 +1936,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) sb->s_op = &ovl_super_operations; if (ofs->config.upperdir) { + struct super_block *upper_sb; + if (!ofs->config.workdir) { pr_err("missing 'workdir'\n"); goto out_err; @@ -1936,6 +1947,16 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) if (err) goto out_err; + upper_sb = ovl_upper_mnt(ofs)->mnt_sb; + if (!ovl_should_sync(ofs)) { + ofs->errseq = errseq_sample(&upper_sb->s_wb_err); + if (errseq_check(&upper_sb->s_wb_err, ofs->errseq)) { + err = -EIO; + pr_err("Cannot mount volatile when upperdir has an unseen error. Sync upperdir fs to clear state.\n"); + goto out_err; + } + } + err = ovl_get_workdir(sb, ofs, &upperpath); if (err) goto out_err; @@ -1943,9 +1964,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) if (!ofs->workdir) sb->s_flags |= SB_RDONLY; - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth; - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran; - + sb->s_stack_depth = upper_sb->s_stack_depth; + sb->s_time_gran = upper_sb->s_time_gran; } oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers); err = PTR_ERR(oe); diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c index 23f475627d07..6e7b8c882045 100644 --- a/fs/overlayfs/util.c +++ b/fs/overlayfs/util.c @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, kfree(buf); return ERR_PTR(res); } + +/* + * ovl_sync_status() - Check fs sync status for volatile mounts + * + * Returns 1 if this is not a volatile mount and a real sync is required. + * + * Returns 0 if syncing can be skipped because mount is volatile, and no errors + * have occurred on the upperdir since the mount. + * + * Returns -errno if it is a volatile mount, and the error that occurred since + * the last mount. If the error code changes, it'll return the latest error + * code. + */ + +int ovl_sync_status(struct ovl_fs *ofs) +{ + struct vfsmount *mnt; + + if (ovl_should_sync(ofs)) + return 1; + + mnt = ovl_upper_mnt(ofs); + if (!mnt) + return 0; + + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->errseq); +}