Re: [PATCH v2 4/4] mm: try to free swapcache for non-LRU folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/24/26 01:16, Barry Song (Xiaomi) wrote:
> Originally, we unconditionally called lru_add_drain() for write
> swap-in page faults. This might drop the reference held by the per-CPU
> LRU cache if the folio happened to reside there. However, there was no
> guarantee that the folio was actually cached on the current CPU.
> 
> Now that lru_add_drain() has been removed, we have lost one
> opportunity to drop a reference held by the LRU cache. We could
> instead incorporate that possibility into the condition evaluated by
> should_try_to_free_swap().
> 
> Suggested-by: Kairui Song <ryncsn@gmail.com>
> Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
> ---
>  mm/memory.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 2983a6baf474..14577c67c61a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5087,8 +5087,11 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  	 * Remove the swap entry and conditionally try to free up the swapcache.
>  	 * Do it after mapping, so raced page faults will likely see the folio
>  	 * in swap cache and wait on the folio lock.
> +	 * Assume non-LRU folios may be queued in the LRU cache, which contributes
> +	 * an additional reference to the folio.
>  	 */
> -	if (should_try_to_free_swap(si, folio, vma, nr_pages, vmf->flags))
> +	if (should_try_to_free_swap(si, folio, vma, nr_pages +
> +			!folio_test_lru(folio), vmf->flags))
>  		folio_free_swap(folio);
>  
>  	folio_unlock(folio);

Hm, in wp_can_reuse_anon_folio() we'll try dropping the swapcache ourselves.

So I wonder if we still need that handling ("If we want to map a page that's in
the swapcache writable, we ...") at all?


Ahh, I see the problem now:

commit 4b34f1d82c6549837b2061096dea249e881a4495
Author: Kairui Song <kasong@tencent.com>
Date:   Sat Dec 20 03:43:35 2025 +0800

    mm, swap: free the swap cache after folio is mapped

    Currently, we remove the folio from the swap cache and free the swap cache
    before mapping the PTE.  To reduce repeated faults due to parallel swapins
    of the same PTE, change it to remove the folio from the swap cache after
    it is mapped.  So new faults from the swap PTE will be much more likely to
    see the folio in the swap cache and wait on it.

    This does not eliminate all swapin races: an ongoing swapin fault may
    still see an empty swap cache.  That's harmless, as the PTE is changed
    before the swap cache is cleared, so it will just return and not trigger
    any repeated faults.  This does help to reduce the chance.


That changed that behavior such that we *must* now always fallback to do_wp_page().

What a mess (I didn't ack)

-- 
Cheers,

David