Re: [PATCH hotfix] mm/migrate: fix kernel BUG at mm/compaction.c:2761!

[Zi Yan]

On 8 Jul 2024, at 11:52, Peter Xu wrote:

> On Tue, Jun 11, 2024 at 10:06:20PM -0700, Hugh Dickins wrote:
>> 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++;
>>  					stats->nr_thp_split += is_thp;
>>  					stats->nr_split++;
>>  					continue;
>> -- 
>> 2.35.3
>>
>>
>
> We probably hit the same issue in our testbeds, but in the other
> migrate_misplaced_folio() path, which contains the BUG_ON() rather than
> VM_BUG_ON().  Looks like this patch can also fix that.
>
> When looking at that, I wonder whether we overlooked one more spot where we
> mostly always use putback_movable_pages() for migrate failures, but didn't
> in migrate_misplaced_folio().  I feel like it was overlooked but want to
> check with all of you here, as I do think the folio can already be split
> when reaching here too. So I wonder whether below would make sense as a fix
> from that POV.
>
> ===8<===
> 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))
> +               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))
> ===8<===

If the original folio is large and split without migrating all subpages,
not migrated sub pages will be left on migratepages list. list_del(&folio->lru)
can remove the first subpage from a wrong list, if it is migrated, and loses
the rest. It is not a problem before, since MR_NUMA_MISPLACED prevents the
folio from being split.

The fix looks good to me.

--
Best Regards,
Yan, Zi

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