linux-kernel - Re: [PATCH v3 4/4] mm: swap: Swap-out small-sized THP without splitting

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Message-ID: <CAGsJ_4w8YWMFjWu2i5NhbOA-pfemvzCHt4hB7rWiOpY63GVWSA@mail.gmail.com>
Date: Tue, 5 Mar 2024 22:54:31 +1300
From: Barry Song <21cnbao@...il.com>
To: Ryan Roberts <ryan.roberts@....com>
Cc: akpm@...ux-foundation.org, david@...hat.com, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, mhocko@...e.com, shy828301@...il.com, 
	wangkefeng.wang@...wei.com, willy@...radead.org, xiang@...nel.org, 
	ying.huang@...el.com, yuzhao@...gle.com, chrisl@...nel.org, surenb@...gle.com, 
	hanchuanhua@...o.com
Subject: Re: [PATCH v3 4/4] mm: swap: Swap-out small-sized THP without splitting

On Tue, Mar 5, 2024 at 10:00 PM Ryan Roberts <ryan.roberts@....com> wrote:
>
> Hi Barry,
>
> On 18/02/2024 23:40, Barry Song wrote:
> > On Tue, Feb 6, 2024 at 1:14 AM Ryan Roberts <ryan.roberts@....com> wrote:
> >>
> >> On 05/02/2024 09:51, Barry Song wrote:
> >>> +Chris, Suren and Chuanhua
> >>>
> >>> Hi Ryan,
> [...]
> >>
> >
> > Hi Ryan,
> > I am running into some races especially while enabling large folio swap-out and
> > swap-in both. some of them, i am still struggling with the detailed
> > timing how they
> > are happening.
> > but the below change can help remove those bugs which cause corrupted data.
>
> I'm getting quite confused with all the emails flying around on this topic. Here
> you were reporting a data corruption bug and your suggested fix below is the one
> you have now posted at [1]. But in the thread at [1] we concluded that it is not
> fixing a functional correctness issue, but is just an optimization in some
> corner cases. So does the corruption issue still manifest? Did you manage to
> root cause it? Is it a problem with my swap-out series or your swap-in series,
> or pre-existing?

Hi Ryan,

It is not a problem of your swap-out series, but a problem of my swap-in
series. The bug in swap-in series is triggered by the skipped PTEs in the
thread[1], but my swap-in code should still be able to cope with this situation
and survive it -  a large folio might be partially but not completely unmapped
after try_to_unmap_one(). I actually replied to you and explained all
the details here[2], but guess you missed it :-)

[1] https://lore.kernel.org/linux-mm/20240304103757.235352-1-21cnbao@gmail.com/
[2] https://lore.kernel.org/linux-mm/CAGsJ_4zdh5kOG7QP4UDaE-wmLFiTEJC2PX-_LxtOj=QrZSvkCA@mail.gmail.com/

apology this makes you confused.

>
> [1] https://lore.kernel.org/linux-mm/20240304103757.235352-1-21cnbao@gmail.com/
>
> Thanks,
> Ryan
>

Thanks
Barry