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

[Peter Xu <peterx@redhat.com>]

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@google.com>
> ---
> 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<===

Thanks,

-- 
Peter Xu