mm/gup: reliable R/O long-term pinning in COW mappings

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.

Link: https://lkml.kernel.org/r/20221116102659.70287-10-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 3d381b26d5531..9d97c9a2a15df 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6197,7 +6197,8 @@ 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);
@@ -6209,7 +6210,7 @@ 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;
 	}
@@ -6338,7 +6339,7 @@ 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;