On 27.02.24 08:04, Lance Yang wrote:
Previously, in folio_pte_batch(), only the upper boundary of the
folio was checked using '>=' for comparison. This led to
incorrect behavior when the next PFN exceeded the lower boundary
of the folio, especially in corner cases where the next PFN might
fall into a different folio.
Which commit does this fix?
The introducing commit (f8d937761d65c87e9987b88ea7beb7bddc333a0e) is
already in mm-stable, so we would need a Fixes: tag. Unless, Ryan's
changes introduced a problem.
BUT
I don't see what is broken. :)
Can you please give an example/reproducer?
We know that the first PTE maps the folio. By incrementing the PFN using
pte_next_pfn/pte_advance_pfn, we cannot suddenly get a lower PFN.
So how would pte_advance_pfn(folio_start_pfn + X) suddenly give us a PFN
lower than folio_start_pfn?
Note that we are not really concerned about any kind of
pte_advance_pfn() overflow that could generate PFN=0. I convinces myself
that that that is something we don't have to worry about.
[I also thought about getting rid of the pte_pfn(pte) >= folio_end_pfn
and instead limiting end_ptep. But that requires more work before the
loop and feels more like a micro-optimization.]
Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
---
mm/memory.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index 642b4f2be523..e5291d1e8c37 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -986,12 +986,15 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
bool *any_writable)
{
- unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
+ unsigned long folio_start_pfn, folio_end_pfn;
const pte_t *end_ptep = start_ptep + max_nr;
pte_t expected_pte, *ptep;
bool writable;
int nr;
+ folio_start_pfn = folio_pfn(folio);
+ folio_end_pfn = folio_start_pfn + folio_nr_pages(folio);
+
if (any_writable)
*any_writable = false;
@@ -1015,7 +1018,7 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
* corner cases the next PFN might fall into a different
* folio.
*/
- if (pte_pfn(pte) >= folio_end_pfn)
+ if (pte_pfn(pte) >= folio_end_pfn || pte_pfn(pte) < folio_start_pfn)
break;
if (any_writable)
--
Cheers,
David / dhildenb
