[RFCv2,0/3] make '%pD' print full path for file

Background
==========
Linus suggested printing full path for file instead of printing
the components as '%pd'.

Typically, there is no need for printk specifiers to take any real locks
(ie mount_lock or rename_lock). So I introduce a new helper d_path_fast
which is similar to d_path except it doesn't take any seqlock/spinlock.

This series is based on Al Viro's d_path cleanup patches [1] which
lifted the inner lockless loop into a new helper. 

[1] https://lkml.org/lkml/2021/5/18/1260

Test
====
The cases I tested:
1. print '%pD' with full path of ext4 file
2. mount a ext4 filesystem upon a ext4 filesystem, and print the file
   with '%pD'
3. print file path which has more than 256 chars,"-ENAMETOOLONG" will
   be returned
4. print file path with '%32pD'
5. all test_print selftests
   
After this set, I found many lines containing '%pD[234]' should be changed
to '%pD'. I don't want to involve those subsystems in this patch series
before the helper is stable enough.
   
You can get the lines by:
$find fs/ -name \*.[ch] | xargs grep -rn "\%pD[234"

fs/overlayfs/file.c:65: pr_debug("open(%p[%pD2/%c], 0%o) -> (%p, 0%o)\n",
fs/nfs/direct.c:453:    dfprintk(FILE, "NFS: direct read(%pD2, %zd@%Ld)\n",
fs/nfs/direct.c:908:    dfprintk(FILE, "NFS: direct write(%pD2, %zd@%Ld)\n",
fs/nfs/write.c:1371:    dprintk("NFS:       nfs_updatepage(%pD2 %d@%lld)\n",
fs/nfs/nfs4file.c:116:  dprintk("NFS: flush(%pD2)\n", file);
fs/nfs/file.c:69:       dprintk("NFS: open file(%pD2)\n", filp);
fs/nfs/file.c:83:       dprintk("NFS: release(%pD2)\n", filp);
fs/nfs/file.c:117:      dprintk("NFS: llseek file(%pD2, %lld, %d)\n",
fs/nfs/file.c:145:      dprintk("NFS: flush(%pD2)\n", file);
fs/nfs/file.c:166:      dprintk("NFS: read(%pD2, %zu@%lu)\n",
fs/nfs/file.c:188:      dprintk("NFS: mmap(%pD2)\n", file);
fs/nfs/file.c:213:      dprintk("NFS: fsync file(%pD2) datasync %d\n", file, datasync);
fs/nfs/file.c:328:      dfprintk(PAGECACHE, "NFS: write_begin(%pD2(%lu), %u@%lld)\n",
fs/nfs/file.c:360:      dfprintk(PAGECACHE, "NFS: write_end(%pD2(%lu), %u@%lld)\n",
fs/nfs/file.c:551:      dfprintk(PAGECACHE, "NFS: vm_page_mkwrite(%pD2(%lu), offset %lld)\n",
fs/nfs/file.c:621:      dprintk("NFS: write(%pD2, %zu@%Ld)\n",
fs/nfs/file.c:803:      dprintk("NFS: lock(%pD2, t=%x, fl=%x, r=%lld:%lld)\n",
fs/nfs/file.c:841:      dprintk("NFS: flock(%pD2, t=%x, fl=%x)\n",
fs/nfs/dir.c:111:       dfprintk(FILE, "NFS: open dir(%pD2)\n", filp);
fs/nfs/dir.c:456:                                               pr_notice("NFS: directory %pD2 contains a readdir loop."
fs/nfs/dir.c:1084:      dfprintk(FILE, "NFS: readdir(%pD2) starting at cookie %llu\n",
fs/nfs/dir.c:1158:      dfprintk(FILE, "NFS: readdir(%pD2) returns %d\n", file, res);
fs/nfs/dir.c:1166:      dfprintk(FILE, "NFS: llseek dir(%pD2, %lld, %d)\n",
fs/nfs/dir.c:1208:      dfprintk(FILE, "NFS: fsync dir(%pD2) datasync %d\n", filp, datasync);
fs/nfsd/nfs4state.c:2439:         seq_printf(s, "filename: \"%pD2\"", f->nf_file);
fs/exec.c:817:          pr_warn_once("process '%pD4' started with executable stack\n",
fs/iomap/direct-io.c:429:               pr_warn_ratelimited("Direct I/O collision with buffered writes! File: %pD4 Comm: %.20s\n",
fs/ioctl.c:81:          pr_warn_ratelimited("[%s/%d] FS: %s File: %pD4 would truncate fibmap result\n",
fs/read_write.c:425:            "kernel %s not supported for file %pD4 (pid: %d comm: %.20s)\n",
fs/splice.c:754:                "splice %s not supported for file %pD4 (pid: %d comm: %.20s)\n",
fs/afs/mntpt.c:64:      _enter("%p,%p{%pD2}", inode, file, file);

Changelog:
v2: 
- implement new d_path_fast based on Al Viro's patches
- add check_pointer check (by Petr)
- change the max full path size to 256 in stack space
v1: https://lkml.org/lkml/2021/5/8/122

Jia He (3):
  fs: introduce helper d_path_fast()
  lib/vsprintf.c: make %pD print full path for file
  s390/hmcdrv: remove the redundant directory path in debug message

 Documentation/core-api/printk-formats.rst |  5 +++--
 drivers/s390/char/hmcdrv_dev.c            | 10 +++++-----
 fs/d_path.c                               | 21 +++++++++++++++++++++
 include/linux/dcache.h                    |  1 +
 lib/vsprintf.c                            | 21 +++++++++++++++++----
 5 files changed, 47 insertions(+), 11 deletions(-)