Re: [PATCH mm-unstable v1 09/20] mm/gup: reliable R/O long-term pinning in COW mappings

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