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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue Oct 13, 2026 at 3:17 PM EDT, Pedro Falcato wrote: > 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, leadi= ng > 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 drop= s > the "clean" anon folio: > > T0 T1 > =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D > 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 | > > Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD") > Cc: stable@vger.kernel.org > Reported-by: Samarth Narang > Closes: https://lore.kernel.org/linux-mm/20261011132227.413217-1-samarth.= colleges@gmail.com/ > Signed-off-by: Pedro Falcato > --- > mm/madvise.c | 5 +---- > 1 file changed, 1 insertion(+), 4 deletions(-) > LGTM. Acked-by: Zi Yan --=20 Best Regards, Yan, Zi