On 02/15/25 at 01:57am, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
>
> Swap allocator will do swap cache reclaim to recycle HAS_CACHE slots for
> allocation. It initiates the reclaim from the offset to be reclaimed and
> looks up the corresponding folio. The lookup process is lockless, so it's
> possible the folio will be removed from the swap cache and given
> a different swap entry before the reclaim locks the folio. If
> it happens, the reclaim will end up reclaiming an irrelevant folio, and
> return wrong return value.
>
> This shouldn't cause any problem with correctness or stability, but
> it is indeed confusing and unexpected, and will increase fragmentation,
> decrease performance.
>
> Fix this by checking whether the folio is still pointing to the offset
> the allocator want to reclaim before reclaiming it.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> ---
> mm/swapfile.c | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 34baefb000b5..c77ffee4af86 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -210,6 +210,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> int ret, nr_pages;
> bool need_reclaim;
>
> +again:
> folio = filemap_get_folio(address_space, swap_cache_index(entry));
> if (IS_ERR(folio))
> return 0;
> @@ -227,8 +228,16 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> if (!folio_trylock(folio))
> goto out;
>
> - /* offset could point to the middle of a large folio */
> + /*
> + * Offset could point to the middle of a large folio, or folio
> + * may no longer point to the expected offset before it's locked.
> + */
> entry = folio->swap;
> + if (offset < swp_offset(entry) || offset >= swp_offset(entry) + nr_pages) {
> + folio_unlock(folio);
> + folio_put(folio);
> + goto again;
> + }
> offset = swp_offset(entry);
LGTM,
Reviewed-by: Baoquan He <bhe@redhat.com>
While reading the code in __try_to_reclaim_swap(), I am always worried
that entry indexed by offset could be accessed by other users so tht
it doesn't only has cache, because we released the ci->lock and don't
hold any lock during period. It could be me who think too much.