| Message ID | 20221116102659.70287-10-david@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | mm/gup: remove FOLL_FORCE usage from drivers (reliable R/O long-term pinning) | expand |
On Wed, Nov 16, 2022 at 11:26:48AM +0100, David Hildenbrand wrote: > We already support reliable R/O pinning of anonymous memory. However, > assume we end up pinning (R/O long-term) a pagecache page or the shared > zeropage inside a writable private ("COW") mapping. The next write access > will trigger a write-fault and replace the pinned page by an exclusive > anonymous page in the process page tables to break COW: the pinned page no > longer corresponds to the page mapped into the process' page table. > > Now that FAULT_FLAG_UNSHARE can break COW on anything mapped into a > COW mapping, let's properly break COW first before R/O long-term > pinning something that's not an exclusive anon page inside a COW > mapping. FAULT_FLAG_UNSHARE will break COW and map an exclusive anon page > instead that can get pinned safely. > > With this change, we can stop using FOLL_FORCE|FOLL_WRITE for reliable > R/O long-term pinning in COW mappings. > > With this change, the new R/O long-term pinning tests for non-anonymous > memory succeed: > # [RUN] R/O longterm GUP pin ... with shared zeropage > ok 151 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd > ok 152 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with tmpfile > ok 153 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with huge zeropage > ok 154 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB) > ok 155 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB) > ok 156 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with shared zeropage > ok 157 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd > ok 158 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with tmpfile > ok 159 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with huge zeropage > ok 160 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB) > ok 161 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB) > ok 162 Longterm R/O pin is reliable > > Note 1: We don't care about short-term R/O-pinning, because they have > snapshot semantics: they are not supposed to observe modifications that > happen after pinning. > > As one example, assume we start direct I/O to read from a page and store > page content into a file: modifications to page content after starting > direct I/O are not guaranteed to end up in the file. So even if we'd pin > the shared zeropage, the end result would be as expected -- getting zeroes > stored to the file. > > Note 2: For shared mappings we'll now always fallback to the slow path to > lookup the VMA when R/O long-term pining. While that's the necessary price > we have to pay right now, it's actually not that bad in practice: most > FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with > FOLL_FORCE because they tried dealing with COW mappings correctly ... > > Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE, > such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd > populate exclusive anon pages that we can pin. There was a concern that > this could affect the memlock limit of existing setups. > > For example, a VM running with VFIO could run into the memlock limit and > fail to run. However, we essentially had the same behavior already in > commit 17839856fd58 ("gup: document and work around "COW can break either > way" issue") which got merged into some enterprise distros, and there were > not any such complaints. So most probably, we're fine. > > Signed-off-by: David Hildenbrand <david@redhat.com> I don't think my ack is any good for the implementation, but for the driver side semantics this sounds like what we want :-) Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> > --- > include/linux/mm.h | 27 ++++++++++++++++++++++++--- > mm/gup.c | 10 +++++----- > mm/huge_memory.c | 2 +- > mm/hugetlb.c | 7 ++++--- > 4 files changed, 34 insertions(+), 12 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 6bd2ee5872dd..e8cc838f42f9 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -3095,8 +3095,12 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags) > * Must be called with the (sub)page that's actually referenced via the > * page table entry, which might not necessarily be the head page for a > * PTE-mapped THP. > + * > + * If the vma is NULL, we're coming from the GUP-fast path and might have > + * to fallback to the slow path just to lookup the vma. > */ > -static inline bool gup_must_unshare(unsigned int flags, struct page *page) > +static inline bool gup_must_unshare(struct vm_area_struct *vma, > + unsigned int flags, struct page *page) > { > /* > * FOLL_WRITE is implicitly handled correctly as the page table entry > @@ -3109,8 +3113,25 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page) > * Note: PageAnon(page) is stable until the page is actually getting > * freed. > */ > - if (!PageAnon(page)) > - return false; > + if (!PageAnon(page)) { > + /* > + * We only care about R/O long-term pining: R/O short-term > + * pinning does not have the semantics to observe successive > + * changes through the process page tables. > + */ > + if (!(flags & FOLL_LONGTERM)) > + return false; > + > + /* We really need the vma ... */ > + if (!vma) > + return true; > + > + /* > + * ... because we only care about writable private ("COW") > + * mappings where we have to break COW early. > + */ > + return is_cow_mapping(vma->vm_flags); > + } > > /* Paired with a memory barrier in page_try_share_anon_rmap(). */ > if (IS_ENABLED(CONFIG_HAVE_FAST_GUP)) > diff --git a/mm/gup.c b/mm/gup.c > index 5182abaaecde..01116699c863 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -578,7 +578,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, > } > } > > - if (!pte_write(pte) && gup_must_unshare(flags, page)) { > + if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) { > page = ERR_PTR(-EMLINK); > goto out; > } > @@ -2338,7 +2338,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, > goto pte_unmap; > } > > - if (!pte_write(pte) && gup_must_unshare(flags, page)) { > + if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) { > gup_put_folio(folio, 1, flags); > goto pte_unmap; > } > @@ -2506,7 +2506,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr, > return 0; > } > > - if (!pte_write(pte) && gup_must_unshare(flags, &folio->page)) { > + if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) { > gup_put_folio(folio, refs, flags); > return 0; > } > @@ -2572,7 +2572,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr, > return 0; > } > > - if (!pmd_write(orig) && gup_must_unshare(flags, &folio->page)) { > + if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) { > gup_put_folio(folio, refs, flags); > return 0; > } > @@ -2612,7 +2612,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr, > return 0; > } > > - if (!pud_write(orig) && gup_must_unshare(flags, &folio->page)) { > + if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) { > gup_put_folio(folio, refs, flags); > return 0; > } > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 68d00196b519..dec7a7c0eca8 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -1434,7 +1434,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma, > if (pmd_protnone(*pmd) && !gup_can_follow_protnone(flags)) > return NULL; > > - if (!pmd_write(*pmd) && gup_must_unshare(flags, page)) > + if (!pmd_write(*pmd) && gup_must_unshare(vma, flags, page)) > return ERR_PTR(-EMLINK); > > VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 383b26069b33..c3aab6d5b7aa 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -6195,7 +6195,8 @@ static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma, > } > } > > -static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte, > +static inline bool __follow_hugetlb_must_fault(struct vm_area_struct *vma, > + unsigned int flags, pte_t *pte, > bool *unshare) > { > pte_t pteval = huge_ptep_get(pte); > @@ -6207,7 +6208,7 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte, > return false; > if (flags & FOLL_WRITE) > return true; > - if (gup_must_unshare(flags, pte_page(pteval))) { > + if (gup_must_unshare(vma, flags, pte_page(pteval))) { > *unshare = true; > return true; > } > @@ -6336,7 +6337,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, > * directly from any kind of swap entries. > */ > if (absent || > - __follow_hugetlb_must_fault(flags, pte, &unshare)) { > + __follow_hugetlb_must_fault(vma, flags, pte, &unshare)) { > vm_fault_t ret; > unsigned int fault_flags = 0; > > -- > 2.38.1 >
On 11/16/22 11:26, David Hildenbrand wrote: > We already support reliable R/O pinning of anonymous memory. However, > assume we end up pinning (R/O long-term) a pagecache page or the shared > zeropage inside a writable private ("COW") mapping. The next write access > will trigger a write-fault and replace the pinned page by an exclusive > anonymous page in the process page tables to break COW: the pinned page no > longer corresponds to the page mapped into the process' page table. > > Now that FAULT_FLAG_UNSHARE can break COW on anything mapped into a > COW mapping, let's properly break COW first before R/O long-term > pinning something that's not an exclusive anon page inside a COW > mapping. FAULT_FLAG_UNSHARE will break COW and map an exclusive anon page > instead that can get pinned safely. > > With this change, we can stop using FOLL_FORCE|FOLL_WRITE for reliable > R/O long-term pinning in COW mappings. > > With this change, the new R/O long-term pinning tests for non-anonymous > memory succeed: > # [RUN] R/O longterm GUP pin ... with shared zeropage > ok 151 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd > ok 152 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with tmpfile > ok 153 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with huge zeropage > ok 154 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB) > ok 155 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB) > ok 156 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with shared zeropage > ok 157 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd > ok 158 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with tmpfile > ok 159 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with huge zeropage > ok 160 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB) > ok 161 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB) > ok 162 Longterm R/O pin is reliable > > Note 1: We don't care about short-term R/O-pinning, because they have > snapshot semantics: they are not supposed to observe modifications that > happen after pinning. > > As one example, assume we start direct I/O to read from a page and store > page content into a file: modifications to page content after starting > direct I/O are not guaranteed to end up in the file. So even if we'd pin > the shared zeropage, the end result would be as expected -- getting zeroes > stored to the file. > > Note 2: For shared mappings we'll now always fallback to the slow path to > lookup the VMA when R/O long-term pining. While that's the necessary price > we have to pay right now, it's actually not that bad in practice: most > FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with > FOLL_FORCE because they tried dealing with COW mappings correctly ... > > Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE, > such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd > populate exclusive anon pages that we can pin. There was a concern that > this could affect the memlock limit of existing setups. > > For example, a VM running with VFIO could run into the memlock limit and > fail to run. However, we essentially had the same behavior already in > commit 17839856fd58 ("gup: document and work around "COW can break either > way" issue") which got merged into some enterprise distros, and there were > not any such complaints. So most probably, we're fine. > > Signed-off-by: David Hildenbrand <david@redhat.com> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
On 11/16/22 02:26, David Hildenbrand wrote: ... > With this change, the new R/O long-term pinning tests for non-anonymous > memory succeed: > # [RUN] R/O longterm GUP pin ... with shared zeropage > ok 151 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd > ok 152 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with tmpfile > ok 153 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with huge zeropage > ok 154 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB) > ok 155 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB) > ok 156 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with shared zeropage > ok 157 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd > ok 158 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with tmpfile > ok 159 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with huge zeropage > ok 160 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB) > ok 161 Longterm R/O pin is reliable > # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB) > ok 162 Longterm R/O pin is reliable Yes. I was able to reproduce these results, after some minor distractions involving huge pages, don't ask. :) > > Note 1: We don't care about short-term R/O-pinning, because they have > snapshot semantics: they are not supposed to observe modifications that > happen after pinning. > > As one example, assume we start direct I/O to read from a page and store > page content into a file: modifications to page content after starting > direct I/O are not guaranteed to end up in the file. So even if we'd pin > the shared zeropage, the end result would be as expected -- getting zeroes > stored to the file. > > Note 2: For shared mappings we'll now always fallback to the slow path to > lookup the VMA when R/O long-term pining. While that's the necessary price > we have to pay right now, it's actually not that bad in practice: most > FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with > FOLL_FORCE because they tried dealing with COW mappings correctly ... > > Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE, > such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd > populate exclusive anon pages that we can pin. There was a concern that > this could affect the memlock limit of existing setups. > > For example, a VM running with VFIO could run into the memlock limit and > fail to run. However, we essentially had the same behavior already in > commit 17839856fd58 ("gup: document and work around "COW can break either > way" issue") which got merged into some enterprise distros, and there were > not any such complaints. So most probably, we're fine. > > Signed-off-by: David Hildenbrand <david@redhat.com> > --- > include/linux/mm.h | 27 ++++++++++++++++++++++++--- > mm/gup.c | 10 +++++----- > mm/huge_memory.c | 2 +- > mm/hugetlb.c | 7 ++++--- > 4 files changed, 34 insertions(+), 12 deletions(-) > Looks good, Reviewed-by: John Hubbard <jhubbard@nvidia.com> thanks,
diff --git a/include/linux/mm.h b/include/linux/mm.h index 6bd2ee5872dd..e8cc838f42f9 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -3095,8 +3095,12 @@ static inline int vm_fault_to_errno(vm_fault_t vm_fault, int foll_flags) * Must be called with the (sub)page that's actually referenced via the * page table entry, which might not necessarily be the head page for a * PTE-mapped THP. + * + * If the vma is NULL, we're coming from the GUP-fast path and might have + * to fallback to the slow path just to lookup the vma. */ -static inline bool gup_must_unshare(unsigned int flags, struct page *page) +static inline bool gup_must_unshare(struct vm_area_struct *vma, + unsigned int flags, struct page *page) { /* * FOLL_WRITE is implicitly handled correctly as the page table entry @@ -3109,8 +3113,25 @@ static inline bool gup_must_unshare(unsigned int flags, struct page *page) * Note: PageAnon(page) is stable until the page is actually getting * freed. */ - if (!PageAnon(page)) - return false; + if (!PageAnon(page)) { + /* + * We only care about R/O long-term pining: R/O short-term + * pinning does not have the semantics to observe successive + * changes through the process page tables. + */ + if (!(flags & FOLL_LONGTERM)) + return false; + + /* We really need the vma ... */ + if (!vma) + return true; + + /* + * ... because we only care about writable private ("COW") + * mappings where we have to break COW early. + */ + return is_cow_mapping(vma->vm_flags); + } /* Paired with a memory barrier in page_try_share_anon_rmap(). */ if (IS_ENABLED(CONFIG_HAVE_FAST_GUP)) diff --git a/mm/gup.c b/mm/gup.c index 5182abaaecde..01116699c863 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -578,7 +578,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, } } - if (!pte_write(pte) && gup_must_unshare(flags, page)) { + if (!pte_write(pte) && gup_must_unshare(vma, flags, page)) { page = ERR_PTR(-EMLINK); goto out; } @@ -2338,7 +2338,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr, goto pte_unmap; } - if (!pte_write(pte) && gup_must_unshare(flags, page)) { + if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) { gup_put_folio(folio, 1, flags); goto pte_unmap; } @@ -2506,7 +2506,7 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr, return 0; } - if (!pte_write(pte) && gup_must_unshare(flags, &folio->page)) { + if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) { gup_put_folio(folio, refs, flags); return 0; } @@ -2572,7 +2572,7 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr, return 0; } - if (!pmd_write(orig) && gup_must_unshare(flags, &folio->page)) { + if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) { gup_put_folio(folio, refs, flags); return 0; } @@ -2612,7 +2612,7 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr, return 0; } - if (!pud_write(orig) && gup_must_unshare(flags, &folio->page)) { + if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) { gup_put_folio(folio, refs, flags); return 0; } diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 68d00196b519..dec7a7c0eca8 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1434,7 +1434,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma, if (pmd_protnone(*pmd) && !gup_can_follow_protnone(flags)) return NULL; - if (!pmd_write(*pmd) && gup_must_unshare(flags, page)) + if (!pmd_write(*pmd) && gup_must_unshare(vma, flags, page)) return ERR_PTR(-EMLINK); VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) && diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 383b26069b33..c3aab6d5b7aa 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -6195,7 +6195,8 @@ static void record_subpages_vmas(struct page *page, struct vm_area_struct *vma, } } -static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte, +static inline bool __follow_hugetlb_must_fault(struct vm_area_struct *vma, + unsigned int flags, pte_t *pte, bool *unshare) { pte_t pteval = huge_ptep_get(pte); @@ -6207,7 +6208,7 @@ static inline bool __follow_hugetlb_must_fault(unsigned int flags, pte_t *pte, return false; if (flags & FOLL_WRITE) return true; - if (gup_must_unshare(flags, pte_page(pteval))) { + if (gup_must_unshare(vma, flags, pte_page(pteval))) { *unshare = true; return true; } @@ -6336,7 +6337,7 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, * directly from any kind of swap entries. */ if (absent || - __follow_hugetlb_must_fault(flags, pte, &unshare)) { + __follow_hugetlb_must_fault(vma, flags, pte, &unshare)) { vm_fault_t ret; unsigned int fault_flags = 0;
We already support reliable R/O pinning of anonymous memory. However, assume we end up pinning (R/O long-term) a pagecache page or the shared zeropage inside a writable private ("COW") mapping. The next write access will trigger a write-fault and replace the pinned page by an exclusive anonymous page in the process page tables to break COW: the pinned page no longer corresponds to the page mapped into the process' page table. Now that FAULT_FLAG_UNSHARE can break COW on anything mapped into a COW mapping, let's properly break COW first before R/O long-term pinning something that's not an exclusive anon page inside a COW mapping. FAULT_FLAG_UNSHARE will break COW and map an exclusive anon page instead that can get pinned safely. With this change, we can stop using FOLL_FORCE|FOLL_WRITE for reliable R/O long-term pinning in COW mappings. With this change, the new R/O long-term pinning tests for non-anonymous memory succeed: # [RUN] R/O longterm GUP pin ... with shared zeropage ok 151 Longterm R/O pin is reliable # [RUN] R/O longterm GUP pin ... with memfd ok 152 Longterm R/O pin is reliable # [RUN] R/O longterm GUP pin ... with tmpfile ok 153 Longterm R/O pin is reliable # [RUN] R/O longterm GUP pin ... with huge zeropage ok 154 Longterm R/O pin is reliable # [RUN] R/O longterm GUP pin ... with memfd hugetlb (2048 kB) ok 155 Longterm R/O pin is reliable # [RUN] R/O longterm GUP pin ... with memfd hugetlb (1048576 kB) ok 156 Longterm R/O pin is reliable # [RUN] R/O longterm GUP-fast pin ... with shared zeropage ok 157 Longterm R/O pin is reliable # [RUN] R/O longterm GUP-fast pin ... with memfd ok 158 Longterm R/O pin is reliable # [RUN] R/O longterm GUP-fast pin ... with tmpfile ok 159 Longterm R/O pin is reliable # [RUN] R/O longterm GUP-fast pin ... with huge zeropage ok 160 Longterm R/O pin is reliable # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (2048 kB) ok 161 Longterm R/O pin is reliable # [RUN] R/O longterm GUP-fast pin ... with memfd hugetlb (1048576 kB) ok 162 Longterm R/O pin is reliable Note 1: We don't care about short-term R/O-pinning, because they have snapshot semantics: they are not supposed to observe modifications that happen after pinning. As one example, assume we start direct I/O to read from a page and store page content into a file: modifications to page content after starting direct I/O are not guaranteed to end up in the file. So even if we'd pin the shared zeropage, the end result would be as expected -- getting zeroes stored to the file. Note 2: For shared mappings we'll now always fallback to the slow path to lookup the VMA when R/O long-term pining. While that's the necessary price we have to pay right now, it's actually not that bad in practice: most FOLL_LONGTERM users already specify FOLL_WRITE, for example, along with FOLL_FORCE because they tried dealing with COW mappings correctly ... Note 3: For users that use FOLL_LONGTERM right now without FOLL_WRITE, such as VFIO, we'd now no longer pin the shared zeropage. Instead, we'd populate exclusive anon pages that we can pin. There was a concern that this could affect the memlock limit of existing setups. For example, a VM running with VFIO could run into the memlock limit and fail to run. However, we essentially had the same behavior already in commit 17839856fd58 ("gup: document and work around "COW can break either way" issue") which got merged into some enterprise distros, and there were not any such complaints. So most probably, we're fine. Signed-off-by: David Hildenbrand <david@redhat.com> --- include/linux/mm.h | 27 ++++++++++++++++++++++++--- mm/gup.c | 10 +++++----- mm/huge_memory.c | 2 +- mm/hugetlb.c | 7 ++++--- 4 files changed, 34 insertions(+), 12 deletions(-)