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List-Subscribe: List-Unsubscribe: On Fri, Jul 04, 2025 at 09:34:17AM +0530, Dev Jain wrote: > Suppose a folio is under migration, and khugepaged is also trying to > collapse it. collapse_pte_mapped_thp() will retrieve the folio from the > page cache via filemap_lock_folio(), thus taking a reference on the folio > and sleeping on the folio lock, since the lock is held by the migration > path. Migration will then fail in > __folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of > such a race happening (leading to migration failure) by bailing out > if we detect a PMD is marked with a migration entry. > > This fixes the migration-shared-anon-thp testcase failure on Apple M3. > > Note that, this is not a "fix" since it only reduces the chance of > interference of khugepaged with migration, wherein both the kernel > functionalities are deemed "best-effort". > > Acked-by: David Hildenbrand > Acked-by: Oscar Salvador > Reviewed-by: Anshuman Khandual > Reviewed-by: Zi Yan > Reviewed-by: Baolin Wang > Signed-off-by: Dev Jain LGTM, so: Reviewed-by: Lorenzo Stoakes > --- > > v2->v3: > - Improve comment (David) > > v1->v2: > - Remove SCAN_PMD_MIGRATION, merge into SCAN_PMD_MAPPED (David, Anshuman) > - Add a comment (Lorenzo) > > v1: > - https://lore.kernel.org/all/20250630044837.4675-1-dev.jain@arm.com/ > > mm/khugepaged.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 1aa7ca67c756..a55fb1dcd224 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -941,6 +941,14 @@ static inline int check_pmd_state(pmd_t *pmd) > > if (pmd_none(pmde)) > return SCAN_PMD_NONE; > + > + /* > + * The folio may be under migration when khugepaged is trying to > + * collapse it. Migration success or failure will eventually end > + * up with a present PMD mapping a folio again. > + */ > + if (is_pmd_migration_entry(pmde)) > + return SCAN_PMD_MAPPED; > if (!pmd_present(pmde)) > return SCAN_PMD_NULL; > if (pmd_trans_huge(pmde)) > -- > 2.30.2 >