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

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Date:   Tue, 21 Sep 2021 12:30:11 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Tiberiu Georgescu <tiberiu.georgescu@...anix.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 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.

> 
> 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.

Thanks,

-- 
Peter Xu