Hugh Dickins <hughd@xxxxxxxxxx> writes:
> I hit the VM_BUG_ON(!list_empty(&cc->migratepages)) in compact_zone();
> and if DEBUG_VM were off, then pages would be lost on a local list.
>
> Our convention is that if migrate_pages() reports complete success (0),
> then the migratepages list will be empty; but if it reports an error or
> some pages remaining, then its caller must putback_movable_pages().
>
> There's a new case in which migrate_pages() has been reporting complete
> success, but returning with pages left on the migratepages list: when
> migrate_pages_batch() successfully split a folio on the deferred list,
> but then the "Failure isn't counted" call does not dispose of them all.
>
> Since that block is expecting the large folio to have been counted as 1
> failure already, and since the return code is later adjusted to success
> whenever the returned list is found empty, the simple way to fix this
> safely is to count splitting the deferred folio as "a failure".
>
> Fixes: 7262f208ca68 ("mm/migrate: split source folio if it is on deferred split list")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
> A hotfix to 6.10-rc, not needed for stable.
>
> mm/migrate.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1654,7 +1654,12 @@ static int migrate_pages_batch(struct list_head *from,
>
> /*
> * The rare folio on the deferred split list should
> - * be split now. It should not count as a failure.
> + * be split now. It should not count as a failure:
> + * but increment nr_failed because, without doing so,
> + * migrate_pages() may report success with (split but
> + * unmigrated) pages still on its fromlist; whereas it
> + * always reports success when its fromlist is empty.
> + *
> * Only check it without removing it from the list.
> * Since the folio can be on deferred_split_scan()
> * local list and removing it can cause the local list
> @@ -1669,6 +1674,7 @@ static int migrate_pages_batch(struct list_head *from,
> if (nr_pages > 2 &&
> !list_empty(&folio->_deferred_list)) {
> if (try_split_folio(folio, split_folios) == 0) {
> + nr_failed++;
It appears better to add
stats->nr_thp_failed++;
too. Otherwise, if migrate_pages_batch() is called via migrate_pages(,
MIGRATE_ASYNC, ), nr_thp_failed will not increase. But if
migrate_pages_batch() is called via migrate_pages(, MIGRATE_SYNC*, ),
nr_thp_failed will increase in migrate_pages_sync() via
stats->nr_thp_failed += astats.nr_thp_split;
That is, they are not consistent. The issue exists since commit
7262f208ca68 ("mm/migrate: split source folio if it is on deferred split
list").
Otherwise, this looks good to me. Thanks!
> stats->nr_thp_split += is_thp;
> stats->nr_split++;
> continue;
--
Best Regards,
Huang, Ying
