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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 reasoning looks good to me. > > 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 removing the unconditional false return after > split_huge_pmd_locked(). Later, we may introduce a true fail early if > split_huge_pmd_locked() does fail. > > 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: > --- > mm/rmap.c | 1 - > 1 file changed, 1 deletion(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 618df3385c8b..eed971568d65 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -2448,7 +2448,6 @@ static bool try_to_migrate_one(struct folio *foli= o, struct vm_area_struct *vma, > if (flags & TTU_SPLIT_HUGE_PMD) { > split_huge_pmd_locked(vma, pvmw.address, > pvmw.pmd, true); > - ret =3D false; > page_vma_mapped_walk_done(&pvmw); > break; > } How about the patch below? It matches the pattern of set_pmd_migration_en= try() below. Basically, continue if the operation is successful, break otherwise. diff --git a/mm/rmap.c b/mm/rmap.c index 618df3385c8b..83cc9d98533e 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -2448,9 +2448,7 @@ static bool try_to_migrate_one(struct folio *folio,= struct vm_area_struct *vma, if (flags & TTU_SPLIT_HUGE_PMD) { split_huge_pmd_locked(vma, pvmw.address, pvmw.pmd, true); - ret =3D false; - page_vma_mapped_walk_done(&pvmw); - break; + continue; } #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION pmdval =3D pmdp_get(pvmw.pmd); -- Best Regards, Yan, Zi