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

[Zi Yan <ziy@nvidia.com>]

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On 2 Jul 2024, at 3:40, Hugh Dickins wrote:

> Even on 6.10-rc6, I've been seeing elusive "Bad page state"s (often on
> flags when freeing, yet the flags shown are not bad: PG_locked had been
> set and cleared??), and VM_BUG_ON_PAGE(page_ref_count(page) == 0)s from
> deferred_split_scan()'s folio_put(), and a variety of other BUG and WARN
> symptoms implying double free by deferred split and large folio migration.
>
> 6.7 commit 9bcef5973e31 ("mm: memcg: fix split queue list crash when large
> folio migration") was right to fix the memcg-dependent locking broken in
> 85ce2c517ade ("memcontrol: only transfer the memcg data for migration"),
> but missed a subtlety of deferred_split_scan(): it moves folios to its own
> local list to work on them without split_queue_lock, during which time
> folio->_deferred_list is not empty, but even the "right" lock does nothing
> to secure the folio and the list it is on.
>
> Fortunately, deferred_split_scan() is careful to use folio_try_get(): so
> folio_migrate_mapping() can avoid the race by folio_undo_large_rmappable()
> while the old folio's reference count is temporarily frozen to 0 - adding
> such a freeze in the !mapping case too (originally, folio lock and
> unmapping and no swap cache left an anon folio unreachable, so no freezing
> was needed there: but the deferred split queue offers a way to reach it).
>
> Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration")
> Signed-off-by: Hugh Dickins <hughd@google.com>
> Cc: stable@vger.kernel.org
> ---
> This patch against 6.10-rc6: Kefeng has commits in the mm-tree which
> which will need adjustment to go over this, but we can both check the
> result.  I have wondered whether just reverting 85ce2c517ade and its
> subsequent fixups would be better: but that would be a bigger job,
> and probably not the right choice.
>
>  mm/memcontrol.c | 11 -----------
>  mm/migrate.c    | 13 +++++++++++++
>  2 files changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 71fe2a95b8bd..8f2f1bb18c9c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -7823,17 +7823,6 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
>
>  	/* Transfer the charge and the css ref */
>  	commit_charge(new, memcg);
> -	/*
> -	 * If the old folio is a large folio and is in the split queue, it needs
> -	 * to be removed from the split queue now, in case getting an incorrect
> -	 * split queue in destroy_large_folio() after the memcg of the old folio
> -	 * is cleared.
> -	 *
> -	 * 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);
>  	old->memcg_data = 0;
>  }
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 20cb9f5f7446..a8c6f466e33a 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -415,6 +415,15 @@ int folio_migrate_mapping(struct address_space *mapping,
>  		if (folio_ref_count(folio) != expected_count)
>  			return -EAGAIN;
>
> +		/* Take off deferred split queue while frozen and memcg set */
> +		if (folio_test_large(folio) &&
> +		    folio_test_large_rmappable(folio)) {
> +			if (!folio_ref_freeze(folio, expected_count))
> +				return -EAGAIN;
> +			folio_undo_large_rmappable(folio);
> +			folio_ref_unfreeze(folio, expected_count);
> +		}
> +

I wonder if the patch below would make the code look better by using
the same freeze/unfreeze pattern like file-backed path. After
reading the emails between you and Baolin and checking the code,
I think the patch looks good to me. Feel free to add
Reviewed-by: Zi Yan <ziy@nvidia.com>

BTW, this subtlety is very error prone, as Matthew, Ryan, and I all
encountered errors because of this[1][2]. Matthew has a good summary
of the subtlety:

"the (undocumented) logic in deferred_split_scan() that a folio
with a positive refcount will not be removed from the list."

[1] https://lore.kernel.org/linux-mm/Ze9EFdFLXQEUVtKl@casper.infradead.org/
[2] https://lore.kernel.org/linux-mm/Ze_P6xagdTbcu1Kz@casper.infradead.org/

diff --git a/mm/migrate.c b/mm/migrate.c
index a8c6f466e33a..afcc0653dcb7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -412,17 +412,15 @@ int folio_migrate_mapping(struct address_space *mapping,

        if (!mapping) {
                /* Anonymous page without mapping */
-               if (folio_ref_count(folio) != expected_count)
+               if (!folio_ref_freeze(folio, expected_count))
                        return -EAGAIN;

                /* Take off deferred split queue while frozen and memcg set */
                if (folio_test_large(folio) &&
-                   folio_test_large_rmappable(folio)) {
-                       if (!folio_ref_freeze(folio, expected_count))
-                               return -EAGAIN;
+                   folio_test_large_rmappable(folio))
                        folio_undo_large_rmappable(folio);
-                       folio_ref_unfreeze(folio, expected_count);
-               }
+
+               folio_ref_unfreeze(folio, expected_count);

                /* No turning back from here */
                newfolio->index = folio->index;


>  		/* No turning back from here */
>  		newfolio->index = folio->index;
>  		newfolio->mapping = folio->mapping;
> @@ -433,6 +442,10 @@ int folio_migrate_mapping(struct address_space *mapping,
>  		return -EAGAIN;
>  	}
>
> +	/* 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);
> +
>  	/*
>  	 * Now we know that no one else is looking at the folio:
>  	 * no turning back from here.
> -- 
> 2.35.3


--
Best Regards,
Yan, Zi

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