linux-kernel - Re: [PATCH v3 4/4] mm: zswap: remove same

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Date: Sat, 13 Apr 2024 22:56:47 +0200
From: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Nhat Pham <nphamcs@...il.com>,
 Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/4] mm: zswap: remove same_filled module params

On 13.04.2024 04:24, Yosry Ahmed wrote:
> These knobs offer more fine-grained control to userspace than needed and
> directly expose/influence kernel implementation; remove them.
> 
> For disabling same_filled handling, there is no logical reason to refuse
> storing same-filled pages more efficiently and opt for compression.
> Scanning pages for patterns may be an argument, but the page contents
> will be read into the CPU cache anyway during compression. Also,
> removing the same_filled handling code does not move the needle
> significantly in terms of performance anyway [1].
> 
> For disabling non_same_filled handling, it was added when the compressed
> pages in zswap were not being properly charged to memcgs, as workloads
> could escape the accounting with compression [2]. This is no longer the
> case after commit f4840ccfca25 ("zswap: memcg accounting"), and using
> zswap without compression does not make much sense.
> 
> [1]https://lore.kernel.org/lkml/CAJD7tkaySFP2hBQw4pnZHJJwe3bMdjJ1t9VC2VJd=khn1_TXvA@mail.gmail.com/
> [2]https://lore.kernel.org/lkml/19d5cdee-2868-41bd-83d5-6da75d72e940@maciej.szmigiero.name/
> 
> Cc: "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Nhat Pham <nphamcs@...il.com>
> Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>
> ---

I think you need to update zswap kernel docs, too.

Thanks,
Maciej