linux-kernel - Re: [PATCH mm-new 2/2] mm/shmem: writeout free swap if swap

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Message-ID: <1d0cbd73-7e4a-a85b-001b-b734d0cb5984@google.com>
Date: Sun, 20 Jul 2025 00:07:46 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Hugh Dickins <hughd@...gle.com>
cc: Andrew Morton <akpm@...ux-foundation.org>, 
    Baolin Wang <baolin.wang@...ux.alibaba.com>, Baoquan He <bhe@...hat.com>, 
    Barry Song <21cnbao@...il.com>, Chris Li <chrisl@...nel.org>, 
    Kairui Song <ryncsn@...il.com>, Kemeng Shi <shikemeng@...weicloud.com>, 
    Shakeel Butt <shakeel.butt@...ux.dev>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: Re: [PATCH mm-new 2/2] mm/shmem: writeout free swap if swap_writeout()
 reactivates

On Wed, 16 Jul 2025, Hugh Dickins wrote:

> If swap_writeout() returns AOP_WRITEPAGE_ACTIVATE (for example, because
> zswap cannot compress and memcg disables writeback), there is no virtue
> in keeping that folio in swap cache and holding the swap allocation:
> shmem_writeout() switch it back to shmem page cache before returning.
> 
> Folio lock is held, and folio->memcg_data remains set throughout, so
> there is no need to get into any memcg or memsw charge complications:
> swap_free_nr() and delete_from_swap_cache() do as much as is needed (but
> beware the race with shmem_free_swap() when inode truncated or evicted).
> 
> Doing the same for an anonymous folio is harder, since it will usually
> have been unmapped, with references to the swap left in the page tables.
> Adding a function to remap the folio would be fun, but not worthwhile
> unless it has other uses, or an urgent bug with anon is demonstrated.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

Tested-by: David Rientjes <rientjes@...gle.com>