linux-kernel - Re: [PATCH v2] mm: shmem: skip swapcache for swapin of synchronous swap device

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Message-ID: <Z3tW-bSC6ze8HtSS@casper.infradead.org>
Date: Mon, 6 Jan 2025 04:07:21 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: akpm@...ux-foundation.org, hughd@...gle.com, david@...hat.com,
	wangkefeng.wang@...wei.com, kasong@...cent.com,
	ying.huang@...ux.alibaba.com, 21cnbao@...il.com,
	ryan.roberts@....com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm: shmem: skip swapcache for swapin of synchronous
 swap device

On Mon, Jan 06, 2025 at 11:46:04AM +0800, Baolin Wang wrote:
> On 2025/1/2 21:10, Matthew Wilcox wrote:
> > On Thu, Jan 02, 2025 at 04:40:17PM +0800, Baolin Wang wrote:
> > > With fast swap devices (such as zram), swapin latency is crucial to applications.
> > > For shmem swapin, similar to anonymous memory swapin, we can skip the swapcache
> > > operation to improve swapin latency.
> > 
> > OK, but now we have more complexity.  Why can't we always skip the
> > swapcache on swapin?
> 
> Skipping swapcache is used to swap-in shmem large folios, avoiding the large
> folios being split. Meanwhile, since the IO latency of syncing swap devices
> is relatively small, it won't cause the IO latency amplification issue.
> 
> But for async swap devices, if we swap-in the large folio one-time, I am
> afraid the IO latency can be amplified. And I remember we still haven't
> reached an agreement here[1], so let's step by step and start with the sync
> swap devices first.

Regardless of whether we choose to swap-in an order-0 or a large folio,
my point is that we should always do it to the pagecache rather than the
swap cache.