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

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Message-ID: <19c2cf15-8789-4977-b149-83b53d2b6abb@arm.com>
Date: Tue, 5 Mar 2024 10:44:46 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.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 05/03/2024 09:54, Barry Song wrote:
> 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(). 

Ahh, understood, thanks!

> I actually replied to you and explained all
> the details here[2], but guess you missed it :-)

I did read that mail, but the first line "They are the same" made me think this
was solving a functional problem. And I still have a very shaky understanding of
parts of the code that I haven't directly worked on, so sometimes some of the
details go over my head - I'll get there eventually!

> 
> [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.

No need to apologise - I appreciate your taking the time to write it all down in
detail. It helps me to learn these areas of the code.

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