linux-kernel - Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

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Date:   Thu, 14 Dec 2023 21:33:34 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Nhat Pham <nphamcs@...il.com>, Yosry Ahmed <yosryahmed@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Chris Li <chriscli@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Vitaly Wool <vitaly.wool@...sulko.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 2/5] mm/zswap: change dstmem size to one page

On 2023/12/14 08:18, Nhat Pham wrote:
> On Wed, Dec 13, 2023 at 3:34 PM Yosry Ahmed <yosryahmed@...gle.com> wrote:
>>
>> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
>> <zhouchengming@...edance.com> wrote:
>>>
>>> Change the dstmem size from 2 * PAGE_SIZE to only one page since
>>> we only need at most one page when compress, and the "dlen" is also
>>> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
>>> we don't wanna store the output in zswap anyway.
>>>
>>> So change it to one page, and delete the stale comment.
>>
>> I couldn't find the history of why we needed 2 * PAGE_SIZE, it would
>> be nice if someone has the context, perhaps one of the maintainers.
> 
> It'd be very nice indeed.
> 
>>
>> One potential reason is that we used to store a zswap header
>> containing the swap entry in the compressed page for writeback
>> purposes, but we don't do that anymore. Maybe we wanted to be able to
>> handle the case where an incompressible page would exceed PAGE_SIZE
>> because of that?
> 
> It could be hmm. I didn't study the old zswap architecture too much,
> but it has been 2 * PAGE_SIZE since the time zswap was first merged
> last I checked.
> I'm not 100% comfortable ACK-ing the undoing of something that looks
> so intentional, but FTR, AFAICT, this looks correct to me.

Right, there is no any history about the reason why we needed 2 pages.
But obviously only one page is needed from the current code and no any
problem found in the kernel build stress testing.

Thanks!