mm: add an explicit smp_wmb() to UFFDIO_CONTINUE

Users of UFFDIO_CONTINUE may reasonably assume that a write memory barrier
is included as part of UFFDIO_CONTINUE.  That is, a user may believe that
all writes it has done to a page that it is now UFFDIO_CONTINUE'ing are
guaranteed to be visible to anyone subsequently reading the page through
the newly mapped virtual memory region.

Today, such a user happens to be correct.  mmget_not_zero(), for example,
is called as part of UFFDIO_CONTINUE (and comes before any PTE updates),
and it implicitly gives us a write barrier.

To be resilient against future changes, include an explicit smp_wmb(). 
While we're at it, optimize the smp_wmb() that is already incidentally
present for the HugeTLB case.

Merely making a syscall does not generally imply the memory ordering
constraints that we need (including on x86).

Link: https://lkml.kernel.org/r/20240307010250.3847179-1-jthoughton@google.com
Signed-off-by: James Houghton <jthoughton@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/hugetlb.c

@@ -6780,11 +6780,20 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
 	}
 
 	/*
-	 * The memory barrier inside __folio_mark_uptodate makes sure that
-	 * preceding stores to the page contents become visible before
-	 * the set_pte_at() write.
+	 * If we just allocated a new page, we need a memory barrier to ensure
+	 * that preceding stores to the page become visible before the
+	 * set_pte_at() write. The memory barrier inside __folio_mark_uptodate
+	 * is what we need.
+	 *
+	 * In the case where we have not allocated a new page (is_continue),
+	 * the page must already be uptodate. UFFDIO_CONTINUE already includes
+	 * an earlier smp_wmb() to ensure that prior stores will be visible
+	 * before the set_pte_at() write.
 	 */
-	__folio_mark_uptodate(folio);
+	if (!is_continue)
+		__folio_mark_uptodate(folio);
+	else
+		WARN_ON_ONCE(!folio_test_uptodate(folio));
 
 	/* Add shared, newly allocated pages to the page cache. */
 	if (vm_shared && !is_continue) {

mm/userfaultfd.c

@@ -845,6 +845,15 @@ ssize_t mfill_atomic_zeropage(struct userfaultfd_ctx *ctx,
 ssize_t mfill_atomic_continue(struct userfaultfd_ctx *ctx, unsigned long start,
 			      unsigned long len, uffd_flags_t flags)
 {
+
+	/*
+	 * A caller might reasonably assume that UFFDIO_CONTINUE contains an
+	 * smp_wmb() to ensure that any writes to the about-to-be-mapped page by
+	 * the thread doing the UFFDIO_CONTINUE are guaranteed to be visible to
+	 * subsequent loads from the page through the newly mapped address range.
+	 */
+	smp_wmb();
+
 	return mfill_atomic(ctx, start, 0, len,
 			    uffd_flags_set_mode(flags, MFILL_ATOMIC_CONTINUE));
 }