On 25.06.24 21:37, Hugh Dickins wrote:
On Tue, 25 Jun 2024, David Hildenbrand wrote:
On 25.06.24 07:00, Hugh Dickins wrote:
Commit "mm: use folio_add_new_anon_rmap() if folio_test_anon(folio)==
false" has extended folio_add_new_anon_rmap() to use on non-exclusive
folios, already visible to others in swap cache and on LRU.
That renders its non-atomic __folio_set_swapbacked() unsafe: it risks
overwriting concurrent atomic operations on folio->flags, losing bits
added or restoring bits cleared. Since it's only used in this risky
way when folio_test_locked and !folio_test_anon, many such races are
excluded; but, for example, isolations by folio_test_clear_lru() are
vulnerable, and setting or clearing active.
It could just use the atomic folio_set_swapbacked(); but this function
does try to avoid atomics where it can, so use a branch instead: just
avoid setting swapbacked when it is already set, that is good enough.
(Swapbacked is normally stable once set: lazyfree can undo it, but
only later, when found anon in a page table.)
This fixes a lot of instability under compaction and swapping loads:
assorted "Bad page"s, VM_BUG_ON_FOLIO()s, apparently even page double
frees - though I've not worked out what races could lead to the latter.
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/rmap.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/mm/rmap.c b/mm/rmap.c
index df1a43295c85..5394c1178bf1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1408,7 +1408,9 @@ void folio_add_new_anon_rmap(struct folio *folio,
struct vm_area_struct *vma,
VM_WARN_ON_FOLIO(folio_test_hugetlb(folio), folio);
VM_BUG_ON_VMA(address < vma->vm_start ||
address + (nr << PAGE_SHIFT) > vma->vm_end, vma);
- __folio_set_swapbacked(folio);
+
+ if (!folio_test_swapbacked(folio))
+ __folio_set_swapbacked(folio);
__folio_set_anon(folio, vma, address, exclusive);
if (likely(!folio_test_large(folio))) {
LGTM.
I'll point out that it's sufficient for a PFN walker to do a tryget + trylock
to cause trouble.
That surprises me. I thought a racer's tryget was irrelevant (touching
a different field) and its trylock not a problem, since "we" hold the
folio lock throughout. If my mental model is too naive there, please
explain in more detail: we all need to understand this better.
Sorry, I was imprecise.
tryget+trylock should indeed not be a problem, tryget+lock would be,
because IIRC folio_wait_bit_common()->folio_set_waiters() would be
messing with folio flags.
--
Cheers,
David / dhildenb
