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: <20250305144823.GB185622@cmpxchg.org>
Date: Wed, 5 Mar 2025 09:48:23 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Nhat Pham <nphamcs@...il.com>,
	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 Wed, Mar 05, 2025 at 06:11:30AM +0000, Yosry Ahmed 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: Johannes Weiner <hannes@...xchg.org>