linux-kernel - Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

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Message-ID: <7e31254d-8889-7e79-50e1-2630bd493d59@redhat.com>
Date:   Thu, 10 Aug 2023 10:37:04 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>, Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Ryan Roberts <ryan.roberts@....com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios

On 10.08.23 05:25, Matthew Wilcox wrote:
> On Wed, Aug 09, 2023 at 05:23:46PM -0400, Peter Xu wrote:
>> Hi, David,
>>
>> Some pure questions below..
>>
>> On Wed, Aug 09, 2023 at 10:32:56AM +0200, David Hildenbrand wrote:
>>> Let's track the total mapcount for all large folios in the first subpage.
>>>
>>> The total mapcount is what we actually want to know in folio_mapcount()
>>> and it is also sufficient for implementing folio_mapped(). This also
>>> gets rid of any "raceiness" concerns as expressed in
>>> folio_total_mapcount().
>>
>> Any more information for that "raciness" described here?
> 
> UTSL.
> 
>          /*
>           * Add all the PTE mappings of those pages mapped by PTE.
>           * Limit the loop to folio_nr_pages_mapped()?
>           * Perhaps: given all the raciness, that may be a good or a bad idea.
>           */
> 

Yes, that comment from Hugh primarily discusses how we could possibly 
optimize the loop, and if relying on folio_nr_pages_mapped() to reduce 
the iterations would be racy. As far as I can see, there are cases where 
"it would be certainly a bad idea" :)


In the other comment in that function, it's also made clear what the 
traditional behavior with PMD-mappable THP was "In the common case, 
avoid the loop when no pages mapped by PTE", which will no longer hold 
with sub-PMD THP.

-- 
Cheers,

David / dhildenb