linux-kernel - Re: [PATCH mm-unstable 2/5] mm: zswap: Use object read/write APIs instead of object mapping APIs

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Message-ID: <CAKEwX=OSKa34Kg2ukzCMpUWiRGgonTHn3pON4LdfJvtexNgu=A@mail.gmail.com>
Date: Wed, 5 Mar 2025 09:35:16 -0800
From: Nhat Pham <nphamcs@...il.com>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, 
	Chengming Zhou <chengming.zhou@...ux.dev>, Minchan Kim <minchan@...nel.org>, 
	Sergey Senozhatsky <senozhatsky@...omium.org>, Herbert Xu <herbert@...dor.apana.org.au>, 
	Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH mm-unstable 2/5] mm: zswap: Use object read/write APIs
 instead of object mapping APIs

On Tue, Mar 4, 2025 at 10:11 PM Yosry Ahmed <yosry.ahmed@...ux.dev> wrote:
>
> Use the new object read/write APIs instead of mapping APIs.
>
> On compress side, zpool_obj_write() is more concise and provides exactly
> what zswap needs to write the compressed object to the zpool, instead of
> map->copy->unmap.
>
> On the decompress side, zpool_obj_read_begin() is sleepable, which
> allows avoiding the memcpy() for zsmalloc and slightly simplifying the
> code by:
> - Avoiding checking if the zpool driver is sleepable, reducing special
>   cases and shrinking the huge comment.
> - Having a single zpool_obj_read_end() call rather than multiple
>   conditional zpool_unmap_handle() calls.
>
> The !virt_addr_valid() case can be removed in the future if the crypto
> API supports kmap addresses or by using kmap_to_page(), completely
> eliminating the memcpy() path in zswap_decompress(). This a step toward
> that. In that spirit, opportunistically make the comment more specific
> about the kmap case instead of generic non-linear addresses. This is the
> only case that needs to be handled in practice, and the generic comment
> makes it seem like a bigger problem that it actually is.
>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@...ux.dev>

Acked-by: Nhat Pham <nphamcs@...il.com>