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+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 is a nice find! > > 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". Thanks for separating this out, appreciated! > > Signed-off-by: Dev Jain > --- > > This patch was part of > https://lore.kernel.org/all/20250625055806.82645-1-dev.jain@arm.com/ > but I have sent it separately on suggestion of Lorenzo, and also because > I plan to send the first two patches after David Hildenbrand's > folio_pte_batch series gets merged. > > mm/khugepaged.c | 12 ++++++++++-- > 1 file changed, 10 insertions(+), 2 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 1aa7ca67c756..99977bb9bf6a 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -31,6 +31,7 @@ enum scan_result { > SCAN_FAIL, > SCAN_SUCCEED, > SCAN_PMD_NULL, > + SCAN_PMD_MIGRATION, > SCAN_PMD_NONE, > SCAN_PMD_MAPPED, > SCAN_EXCEED_NONE_PTE, > @@ -941,6 +942,8 @@ static inline int check_pmd_state(pmd_t *pmd) > > if (pmd_none(pmde)) > return SCAN_PMD_NONE; > + if (is_pmd_migration_entry(pmde)) > + return SCAN_PMD_MIGRATION; With David's suggestions I guess this boils down to simply adding this line. Could we add a quick comment to explain why here? Thanks! > if (!pmd_present(pmde)) > return SCAN_PMD_NULL; > if (pmd_trans_huge(pmde)) > @@ -1502,9 +1505,12 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, > !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE)) > return SCAN_VMA_CHECK; > > - /* Fast check before locking page if already PMD-mapped */ > + /* > + * Fast check before locking folio if already PMD-mapped, or if the > + * folio is under migration > + */ > result = find_pmd_or_thp_or_none(mm, haddr, &pmd); > - if (result == SCAN_PMD_MAPPED) > + if (result == SCAN_PMD_MAPPED || result == SCAN_PMD_MIGRATION) > return result; > > /* > @@ -2716,6 +2722,7 @@ static int madvise_collapse_errno(enum scan_result r) > case SCAN_PAGE_LRU: > case SCAN_DEL_PAGE_LRU: > case SCAN_PAGE_FILLED: > + case SCAN_PMD_MIGRATION: > return -EAGAIN; > /* > * Other: Trying again likely not to succeed / error intrinsic to > @@ -2802,6 +2809,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > goto handle_result; > /* Whitelisted set of results where continuing OK */ > case SCAN_PMD_NULL: > + case SCAN_PMD_MIGRATION: > case SCAN_PTE_NON_PRESENT: > case SCAN_PTE_UFFD_WP: > case SCAN_PAGE_RO: > -- > 2.30.2 >