linux-kernel - Re: [PATCH 2/6] mm/zswap: invalidate zswap entry when swap entry free

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Date: Fri, 2 Feb 2024 00:11:46 +0000
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Chengming Zhou <zhouchengming@...edance.com>
Cc: Nhat Pham <nphamcs@...il.com>, Johannes Weiner <hannes@...xchg.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org
Subject: Re: [PATCH 2/6] mm/zswap: invalidate zswap entry when swap entry free

On Thu, Feb 01, 2024 at 03:49:02PM +0000, Chengming Zhou wrote:
> During testing I found there are some times the zswap_writeback_entry()
> return -ENOMEM, which is not we expected:
> 
> bpftrace -e 'kr:zswap_writeback_entry {@[(int32)retval]=count()}'
> @[-12]: 1563
> @[0]: 277221
> 
> The reason is that __read_swap_cache_async() return NULL because
> swapcache_prepare() failed. The reason is that we won't invalidate
> zswap entry when swap entry freed to the per-cpu pool, these zswap
> entries are still on the zswap tree and lru list.
> 
> This patch moves the invalidation ahead to when swap entry freed
> to the per-cpu pool, since there is no any benefit to leave trashy
> zswap entry on the tree and lru list.
> 
> With this patch:
> bpftrace -e 'kr:zswap_writeback_entry {@[(int32)retval]=count()}'
> @[0]: 259744
> 
> Note: large folio can't have zswap entry for now, so don't bother
> to add zswap entry invalidation in the large folio swap free path.

This makes me slightly nervous. Should we add a comment somewhere just
in case this is missed if someone adds large folio support?

Otherwise the patch itself LGTM.