linux-kernel - Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

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Message-ID: <CAJD7tkaXvm95mRH04OX0KJtiBuTaaDyyJQirbAjUV0B+DjaWJA@mail.gmail.com>
Date: Tue, 3 Sep 2024 15:05:25 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Barry Song <21cnbao@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kairui Song <ryncsn@...il.com>, hanchuanhua@...o.com, 
	Usama Arif <usamaarif642@...il.com>, linux-mm@...ck.org, baolin.wang@...ux.alibaba.com, 
	chrisl@...nel.org, david@...hat.com, hannes@...xchg.org, hughd@...gle.com, 
	kaleshsingh@...gle.com, linux-kernel@...r.kernel.org, mhocko@...e.com, 
	minchan@...nel.org, nphamcs@...il.com, ryan.roberts@....com, 
	senozhatsky@...omium.org, shakeel.butt@...ux.dev, shy828301@...il.com, 
	surenb@...gle.com, v-songbaohua@...o.com, willy@...radead.org, 
	xiang@...nel.org, ying.huang@...el.com, hch@...radead.org
Subject: Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

On Tue, Sep 3, 2024 at 2:36 PM Barry Song <21cnbao@...il.com> wrote:
>
> On Wed, Sep 4, 2024 at 8:08 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
> >
> > On Tue, 3 Sep 2024 11:38:37 -0700 Yosry Ahmed <yosryahmed@...gle.com> wrote:
> >
> > > > [   39.157954] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000007
> > > > [   39.158288] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
> > > > [   39.158634] R13: 0000000000002b9a R14: 0000000000000000 R15: 00007ffd619d5518
> > > > [   39.158998]  </TASK>
> > > > [   39.159226] ---[ end trace 0000000000000000 ]---
> > > >
> > > > After reverting this or Usama's "mm: store zero pages to be swapped
> > > > out in a bitmap", the problem is gone. I think these two patches may
> > > > have some conflict that needs to be resolved.
> > >
> > > Yup. I saw this conflict coming and specifically asked for this
> > > warning to be added in Usama's patch to catch it [1]. It served its
> > > purpose.
> > >
> > > Usama's patch does not handle large folio swapin, because at the time
> > > it was written we didn't have it. We expected Usama's series to land
> > > sooner than this one, so the warning was to make sure that this series
> > > handles large folio swapin in the zeromap code. Now that they are both
> > > in mm-unstable, we are gonna have to figure this out.
> > >
> > > I suspect Usama's patches are closer to land so it's better to handle
> > > this in this series, but I will leave it up to Usama and
> > > Chuanhua/Barry to figure this out :)
>
> I believe handling this in swap-in might violate layer separation.
> `swap_read_folio()` should be a reliable API to call, regardless of
> whether `zeromap` is present. Therefore, the fix should likely be
> within `zeromap` but not this `swap-in`. I’ll take a look at this with
> Usama :-)

I meant handling it within this series to avoid blocking Usama
patches, not within this code. Thanks for taking a look, I am sure you
and Usama will figure out the best way forward :)