linux-kernel - Re: [PATCH v2 1/1] Documentation: update pagemap with shmem exceptions

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Date:   Tue, 21 Sep 2021 17:09:48 +0000
From:   Tiberiu Georgescu <tiberiu.georgescu@...anix.com>
To:     Peter Xu <peterx@...hat.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        "david@...hat.com" <david@...hat.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        Florian Schmidt <flosch@...anix.com>,
        "Carl Waldspurger [C]" <carl.waldspurger@...anix.com>,
        Jonathan Davies <jond@...anix.com>
Subject: Re: [PATCH v2 1/1] Documentation: update pagemap with shmem
 exceptions


> On 21 Sep 2021, at 17:30, Peter Xu <peterx@...hat.com> wrote:
> 
> On Tue, Sep 21, 2021 at 04:08:29PM +0000, Tiberiu Georgescu wrote:
>> Hmmm, so if we put emphasis on the accuracy of swap info, or accuracy in
>> general, we need to use frontswap. Otherwise, mincore() could suffer from
>> race conditions, and mark pages in the swap cache as being present.
> 
> IMHO it's not a race condition, but by design.
> 
> Quotting again from the mincore() man page:
> 
>       The vec argument must point to an array containing at least
>       (length+PAGE_SIZE-1) / PAGE_SIZE bytes.  On return, the least
>       significant bit of each byte will be set if the corresponding page is
>       currently resident in memory, and be clear otherwise.
> 
> I think "within swap cache" does mean that it still resides in memory, so it's
> not violating what it's designed to me, at least from the manpage.

I agree mincore() is doing what it is designed to do. I did not express my
thoughts correctly.
> 
>> 
>> Do you reckon this info (frontswap for mincore) should be present in
>> the pagemap docs? I wouldn't want to bloat the section either.
> 
> I don't think the type of swap matters in this document, but imho mentioning
> mincore() as the alternative to fetch swap is still meaningful as that's what's
> missing for pagemap right now on shmem typed memories.
> 
> Even if it cannot identify some cases between "page presents", "page stays in
> page cache", or "page stays in swap cache", it'll still be good enough to me.

Yeah, it should be good enough mostly. But I feel that providing an exact
algorithm on how to separate the cases is not appropriate anymore,
as there are still exceptions, and the "perfect" algorithm needs frontswap
as a backend, which can be a fairly prohibitive precondition.

Maybe I should just note all these tools down instead and provide a brief
explanation of their usage and their downsides in the context of pagemap
information.

Thanks a lot for all suggestions! I'll cook up a v3 shortly.

Kind regards,
Tibi