linux-kernel - Re: [RFC PATCH v2 1/1] mm/vmscan: move the written-back folios to the tail of LRU after shrinking

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Message-ID: <ZzrA5nXldoE2PWx4@casper.infradead.org>
Date: Mon, 18 Nov 2024 04:21:58 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Barry Song <21cnbao@...il.com>
Cc: Chen Ridong <chenridong@...weicloud.com>, akpm@...ux-foundation.org,
	mhocko@...e.com, hannes@...xchg.org, yosryahmed@...gle.com,
	yuzhao@...gle.com, david@...hat.com, ryan.roberts@....com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	chenridong@...wei.com, wangweiyang2@...wei.com,
	xieym_ict@...mail.com, Chris Li <chrisl@...nel.org>
Subject: Re: [RFC PATCH v2 1/1] mm/vmscan: move the written-back folios to
 the tail of LRU after shrinking

On Mon, Nov 18, 2024 at 05:14:14PM +1300, Barry Song wrote:
> On Mon, Nov 18, 2024 at 5:03 PM Matthew Wilcox <willy@...radead.org> wrote:
> >
> > On Sat, Nov 16, 2024 at 09:16:58AM +0000, Chen Ridong wrote:
> > > 2. In shrink_page_list function, if folioN is THP(2M), it may be splited
> > >    and added to swap cache folio by folio. After adding to swap cache,
> > >    it will submit io to writeback folio to swap, which is asynchronous.
> > >    When shrink_page_list is finished, the isolated folios list will be
> > >    moved back to the head of inactive lru. The inactive lru may just look
> > >    like this, with 512 filioes have been move to the head of inactive lru.
> >
> > I was hoping that we'd be able to stop splitting the folio when adding
> > to the swap cache.  Ideally. we'd add the whole 2MB and write it back
> > as a single unit.
> 
> This is already the case: adding to the swapcache doesn’t require splitting
> THPs, but failing to allocate 2MB of contiguous swap slots will.

Agreed we need to understand why this is happening.  As I've said a few
times now, we need to stop requiring contiguity.  Real filesystems don't
need the contiguity (they become less efficient, but they can scatter a
single 2MB folio to multiple places).

Maybe Chris has a solution to this in the works?