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, 24 insertions, 3 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 686879dbb0bd..d8363ac34a7c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3149,8 +3149,12 @@ 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
@@ -3163,8 +3167,25 @@ 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))