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Message-ID: <da6bad97-18b8-4cd0-9dcc-b60fb20b7a84@linux.alibaba.com>
Date: Tue, 2 Jul 2024 17:25:13 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Nhat Pham <nphamcs@...il.com>, Yang Shi <shy828301@...il.com>,
Zi Yan <ziy@...dia.com>, Barry Song <baohua@...nel.org>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH hotfix] mm: fix crashes from deferred split racing folio
migration
On 2024/7/2 15:40, 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).
Thanks Hugh.
But after reading your analysis, I am concerned that the
folio_undo_large_rmappable() and deferred_split_scan() may still
encounter a race condition with the local list, even with your patch.
Suppose folio A has already been queued into the local list in
deferred_split_scan() by thread A, but fails to 'folio_trylock' and then
releases the reference count. At the same time, folio A can be frozen by
another thread B in folio_migrate_mapping(). In such a case,
folio_undo_large_rmappable() would remove folio A from the local list
without *any* lock protection, creating a race condition with the local
list iteration in deferred_split_scan().
Anyway, I think this patch can still fix some possible races. Feel free
to add:
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
>
> Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: stable@...r.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);
> + }
> +
> /* 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.
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