Re: [PATCH] mm/migrate: Putback split folios when numa hint migration fails

Baolin Wang
2024-07-09T01:48:59Z

On 2024/7/9 05:55, Peter Xu wrote:
> This issue is not from any report yet, but by code observation only.
>
> This is yet another fix besides Hugh's patch [1] but on relevant code path,
> where eager split of folio can happen if the folio is already on deferred
> list during a folio migration.
>
> Here the issue is NUMA path (migrate_misplaced_folio()) may start to
> encounter such folio split now even with MR_NUMA_MISPLACED hint applied.
> Then when migrate_pages() didn't migrate all the folios, it's possible the
> split small folios be put onto the list instead of the original folio.
> Then putting back only the head page won't be enough.
>
> Fix it by putting back all the folios on the list.
>
> [1] https://lore.kernel.org/all/46c948b4-4dd8-6e03-4c7b-ce4e81cfa536@google.com/
>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Fixes: 7262f208ca68 ("mm/migrate: split source folio if it is on deferred split list")
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---

Good catch. With fixing the building issue pointed by Andrew, please
feel free to add:
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

>
> Don't need to copy stable if this can still hit 6.10.. Only smoke tested.
> ---
> mm/migrate.c | 10 ++--------
> 1 file changed, 2 insertions(+), 8 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e10d2445fbd8..20da2595527a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2615,14 +2615,8 @@ int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma,
> nr_remaining = migrate_pages(&migratepages, alloc_misplaced_dst_folio,
> NULL, node, MIGRATE_ASYNC,
> MR_NUMA_MISPLACED, &nr_succeeded);
> - if (nr_remaining) {
> - if (!list_empty(&migratepages)) {
> - list_del(&folio->lru);
> - node_stat_mod_folio(folio, NR_ISOLATED_ANON +
> - folio_is_file_lru(folio), -nr_pages);
> - folio_putback_lru(folio);
> - }
> - }
> + if (nr_remaining && !list_empty(&migratepages))

Nit: you can drop the '!list_empty(&migratepages)' validation, since
putback_movable_pages() can handle this unusual case.

> + putback_movable_pages(&migratepages);
> if (nr_succeeded) {
> count_vm_numa_events(NUMA_PAGE_MIGRATE, nr_succeeded);
> if (!node_is_toptier(folio_nid(folio)) && node_is_toptier(node))