linux-kernel - Re: [PATCH 04/19] mm, swap: always try to free swap cache for SWP_SYNCHRONOUS

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Message-ID: <CAGsJ_4xOd0CJLZvzvZu9mRLjVvV0OSKE8NcBAYe3wvHFtLjWyA@mail.gmail.com>
Date: Tue, 4 Nov 2025 12:19:37 +0800
From: Barry Song <21cnbao@...il.com>
To: Kairui Song <ryncsn@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	Baoquan He <bhe@...hat.com>, Chris Li <chrisl@...nel.org>, Nhat Pham <nphamcs@...il.com>, 
	Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosry.ahmed@...ux.dev>, 
	David Hildenbrand <david@...hat.com>, Youngjun Park <youngjun.park@....com>, 
	Hugh Dickins <hughd@...gle.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>, 
	"Huang, Ying" <ying.huang@...ux.alibaba.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, linux-kernel@...r.kernel.org, 
	Kairui Song <kasong@...cent.com>
Subject: Re: [PATCH 04/19] mm, swap: always try to free swap cache for
 SWP_SYNCHRONOUS_IO devices

On Wed, Oct 29, 2025 at 11:59 PM Kairui Song <ryncsn@...il.com> wrote:
>
> From: Kairui Song <kasong@...cent.com>
>
> Now SWP_SYNCHRONOUS_IO devices are also using swap cache. One side
> effect is that a folio may stay in swap cache for a longer time due to
> lazy freeing (vm_swap_full()). This can help save some CPU / IO if folios
> are being swapped out very frequently right after swapin, hence improving
> the performance. But the long pinning of swap slots also increases the
> fragmentation rate of the swap device significantly, and currently,
> all in-tree SWP_SYNCHRONOUS_IO devices are RAM disks, so it also
> causes the backing memory to be pinned, increasing the memory pressure.
>
> So drop the swap cache immediately for SWP_SYNCHRONOUS_IO devices
> after swapin finishes. Swap cache has served its role as a
> synchronization layer to prevent any parallel swapin from wasting
> CPU or memory allocation, and the redundant IO is not a major concern
> for SWP_SYNCHRONOUS_IO devices.
>
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>  mm/memory.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 9a43d4811781..78457347ae60 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4359,12 +4359,21 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
>         return 0;
>  }
>
> -static inline bool should_try_to_free_swap(struct folio *folio,
> +static inline bool should_try_to_free_swap(struct swap_info_struct *si,
> +                                          struct folio *folio,
>                                            struct vm_area_struct *vma,
>                                            unsigned int fault_flags)
>  {
>         if (!folio_test_swapcache(folio))
>                 return false;
> +       /*
> +        * Try to free swap cache for SWP_SYNCHRONOUS_IO devices.
> +        * Redundant IO is unlikely to be an issue for them, but a
> +        * slot being pinned by swap cache may cause more fragmentation
> +        * and delayed freeing of swap metadata.
> +        */

I don’t like the claim about “redundant I/O” — it sounds misleading. Those
I/Os are not redundant; they are simply saved by swapcache, which prevents
some swap-out I/O when a recently swap-in folio is swapped out again.

So, could we make it a bit more specific in both the comment and the commit
message?

Thanks
Barry