On 26/03/2024 16:30, David Hildenbrand wrote:

On 15.02.24 13:17, Ryan Roberts wrote:

quoted

Gup needs to read ptes locklessly, so it uses ptep_get_lockless().
However, the returned access and dirty bits are unimportant so let's
switch over to ptep_get_lockless_norecency().

The wrinkle is that gup needs to check that the pte hasn't changed once
it has pinned the folio following this model:

     pte = ptep_get_lockless_norecency(ptep)
     ...
     if (!pte_same(pte, ptep_get_lockless(ptep)))
             // RACE!
     ...

And now that pte may not contain correct access and dirty information,
the pte_same() comparison could spuriously fail. So let's introduce a
new pte_same_norecency() helper which will ignore the access and dirty
bits when doing the comparison.

Note that previously, ptep_get() was being used for the comparison; this
is technically incorrect because the PTL is not held. I've also
converted the comparison to use the preferred pmd_same() helper instead
of doing a raw value comparison.

As a side-effect, this new approach removes the possibility of
concurrent read/write to the page causing a spurious fast gup failure,
because the access and dirty bits are no longer used in the comparison.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---

[...]

quoted

  #ifndef __HAVE_ARCH_PTE_UNUSED
  /*
   * Some architectures provide facilities to virtualization guests

diff --git a/mm/gup.c b/mm/gup.c
index df83182ec72d..0f96d0a5ec09 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -2576,7 +2576,7 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp,

unsigned long addr,
      if (!ptep)
          return 0;
      do {
-        pte_t pte = ptep_get_lockless(ptep);
+        pte_t pte = ptep_get_lockless_norecency(ptep);
          struct page *page;
          struct folio *folio;

@@ -2617,8 +2617,9 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp,

unsigned long addr,
              goto pte_unmap;
          }

-        if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
-            unlikely(pte_val(pte) != pte_val(ptep_get(ptep)))) {
+        if (unlikely(!pmd_same(pmd, *pmdp)) ||
+            unlikely(!pte_same_norecency(pte,
+                    ptep_get_lockless_norecency(ptep)))) {
              gup_put_folio(folio, 1, flags);
              goto pte_unmap;

We pass the pte into pte_access_permitted(). It would be good to mention that
you checked all implementations.

TBH, I hadn't; I decided that since the "inaccurate access/dirty bits" was only
possible on arm64, then only arm64's implementation needed checking. But given
your comment, I just had a quick look at all impls. I didn't spot any problems
where any impl needs the access/dirty bits. I'll add this to the commit log.

Acked-by: David Hildenbrand <redacted>

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel