Re: [PATCH hotfix] mm: fix crashes from deferred split racing folio migration

[Kefeng Wang]

On 2024/7/4 11:21, Hugh Dickins wrote:
> On Wed, 3 Jul 2024, Andrew Morton wrote:
> > On Tue, 2 Jul 2024 00:40:55 -0700 (PDT) Hugh Dickins <hughd@xxxxxxxxxx> wrote:
...
> And perhaps a conflict with another one of Kefeng's, which deletes a hunk
> in mm/migrate.c just above where I add a hunk: and that's indeed how it
> should end up, hunk deleted by Kefeng, hunk added by me.
>
> > --- mm/memcontrol.c~mm-refactor-folio_undo_large_rmappable
> > +++ mm/memcontrol.c
> > @@ -7832,8 +7832,7 @@ void mem_cgroup_migrate(struct folio *ol
> >   	 * In addition, the old folio is about to be freed after migration, so
> >   	 * removing from the split queue a bit earlier seems reasonable.
> >   	 */
> > -	if (folio_test_large(old) && folio_test_large_rmappable(old))
> > -		folio_undo_large_rmappable(old);
> > +	folio_undo_large_rmappable(old);
> >   	old->memcg_data = 0;
> >   }
> >
> > I'm resolving this by simply dropping the above hunk.  So Kefeng's
> > patch is now as below.  Please check.
>
> Checked, and that is correct, thank you Andrew.  Correct, but not quite
> complete: because I'm sure that if Kefeng had written his patch after
> mine, he would have made the equivalent change in mm/migrate.c:

Yes,

> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -443,8 +443,7 @@ int folio_migrate_mapping(struct address_space *mapping,
>   	}
>   
>   	/* Take off deferred split queue while frozen and memcg set */
> -	if (folio_test_large(folio) && folio_test_large_rmappable(folio))
> -		folio_undo_large_rmappable(folio);
> +	folio_undo_large_rmappable(folio);
>   
>   	/*
>   	 * Now we know that no one else is looking at the folio:
>
> But there's no harm done if you push out a tree without that additional
> mod: we can add it as a fixup afterwards, it's no more than a cleanup.
Maybe we could convert to __folio_undo_large_rmappable() for !maping 
part, which will avoid unnecessary freeze/unfreeze for empty deferred
list.

diff --git a/mm/migrate.c b/mm/migrate.c
index ca65f03acb31..e6af9c25c25b 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -412,10 +412,11 @@ static int __folio_migrate_mapping(struct 
address_space *mapping,
        if (!mapping) {
                /* Take off deferred split queue while frozen and memcg 
set */
                if (folio_test_large(folio) &&
-                   folio_test_large_rmappable(folio)) {
+                   folio_test_large_rmappable(folio) &&
+                   !data_race(list_empty(&folio->_deferred_list))) {
                        if (!folio_ref_freeze(folio, expected_count))
                                return -EAGAIN;
-                       folio_undo_large_rmappable(folio);
+                       __folio_undo_large_rmappable(folio);
                        folio_ref_unfreeze(folio, expected_count);
                }


> (I'm on the lookout for an mm.git update, hope to give it a try when it
> appears.)
>
> Thanks,
> Hugh