[PATCH 6.1.y] mm/madvise: atomically clear the young bit on THP MADV_COLD

[Sasha Levin <sashal@kernel.org>]

From: Pedro Falcato <pfalcato@suse.de>

[ Upstream commit 246c8cc33438a71c28f9b86baa492d9a0a130316 ]

As-is, madvise_cold_or_pageout_pte_range() discards the hardware young bit
by invalidating the PMD, doing pmd_mkold() and subsequently writing to the
pmd again.  However, it does so in a racy manner, looking at a
possibly-stale snapshot.  It's entirely possible that a write elsewhere
set the hardware D bit, which this logic now loses.

Fix it by using pmdp_test_and_clear_young().  This is an atomic
modification of the PMD entry, and thus safe against any possible racing
writes.  As a (nice) side-effect, this elides one non-required TLB flush
in pmdp_invalidate().

This bug can interact with the swapping system in complicated ways,
leading to data loss.  For instance, if a folio is marked lazyfree, but
was later read from, and then MADV_COLD'd while racing a write to that
same THP, dropping the dirty bit may lead to data loss as
reclaim/MADV_PAGEOUT drops the "clean" anon folio:

T0                                T1
================================================================
MADV_FREE(thp)			|
  folio marked lazyfree		|
read(thp)			|
  PMD marked young by hardware	|
				| MADV_COLD(thp)
write(thp)			|   if (pmd_young(pmd)) {
  PMD marked dirty by hardware	|     set(pmdp, pmd_mkold(pmd));
MADV_PAGEOUT/reclaim		|
  dirty bit may or may not be   |
  set				|
    reclaim/pageout may drop    |
    the folio			|

Link: https://lore.kernel.org/20261013191719.95502-1-pfalcato@suse.de
Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD")
Signed-off-by: Pedro Falcato <pfalcato@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: Samarth Narang <samarth.colleges@gmail.com>
Closes: https://lore.kernel.org/linux-mm/20261011132227.413217-1-samarth.colleges@gmail.com/
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Acked-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: <stable@vger.kernel.org>
[ sashal: Retained pmd_young(orig_pmd) without the !pageout guard because the corresponding reclaim optimization is absent. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 mm/madvise.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index b271cc1b48edb..5640bacba5384 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -379,10 +379,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
 		}
 
 		if (pmd_young(orig_pmd)) {
-			pmdp_invalidate(vma, addr, pmd);
-			orig_pmd = pmd_mkold(orig_pmd);
-
-			set_pmd_at(mm, addr, pmd, orig_pmd);
+			pmdp_test_and_clear_young(vma, addr, pmd);
 			tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
 		}
 
-- 
2.53.0