Re: [PATCH 1/7] mm, swap: avoid reclaiming irrelevant swap cache

[Baoquan He <bhe@redhat.com>]

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.