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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 3 Feb 2026, at 19:42, Wei Yang wrote: > Commit 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and > split_huge_pmd_locked()") return false unconditionally after > split_huge_pmd_locked() which may fail early during try_to_migrate() fo= r > shared thp. This will lead to unexpected folio split failure. > > One way to reproduce: > > Create an anonymous thp range and fork 512 children, so we have a > thp shared mapped in 513 processes. Then trigger folio split with > /sys/kernel/debug/split_huge_pages debugfs to split the thp folio t= o > order 0. > > Without the above commit, we can successfully split to order 0. > With the above commit, the folio is still a large folio. > > The reason is the above commit return false after split pmd > unconditionally in the first process and break try_to_migrate(). > > The tricky thing in above reproduce method is current debugfs interface= > leverage function split_huge_pages_pid(), which will iterate the whole > pmd range and do folio split on each base page address. This means it > will try 512 times, and each time split one pmd from pmd mapped to pte > mapped thp. If there are less than 512 shared mapped process, > the folio is still split successfully at last. But in real world, we > usually try it for once. > > This patch fixes this by restart page_vma_mapped_walk() after > split_huge_pmd_locked(). Because split_huge_pmd_locked() may fall back = to > (freeze =3D false) if folio_try_share_anon_rmap_pmd() fails and the PMD= is > just split instead of split to migration entry. Restart > page_vma_mapped_walk() and let try_to_migrate_one() try on each PTE > again and fail try_to_migrate() early if it fails. > > Signed-off-by: Wei Yang > Fixes: 60fbb14396d5 ("mm/huge_memory: adjust try_to_migrate_one() and s= plit_huge_pmd_locked()") > Cc: Gavin Guo > Cc: "David Hildenbrand (Red Hat)" > Cc: Zi Yan > Cc: Baolin Wang > Cc: Lance Yang > Cc: > > --- > v2: > * restart page_vma_mapped_walk() after split_huge_pmd_locked() > --- > mm/rmap.c | 11 ++++++++--- > 1 file changed, 8 insertions(+), 3 deletions(-) > Reviewed-by: Zi Yan Best Regards, Yan, Zi