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

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Date:   Thu, 10 Aug 2023 10:59:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     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>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        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 09.08.23 23:23, Peter Xu wrote:
> Hi, David,
> 
> Some pure questions below..

Hi Peter,

thanks for having a look!

[...]

>> With sub-PMD THP becoming more important and things looking promising
>> that we will soon get support for such anon THP, we want to avoid looping
>> over all pages of a folio just to calculate the total mapcount. Further,
>> we may soon want to use the total mapcount in other context more
>> frequently, so prepare for reading it efficiently and atomically.
> 
> Any (perhaps existing) discussion on reduced loops vs added atomic
> field/ops?

So far it's not been raised as a concern, so no existing discussion.

For order-0 pages the behavior is unchanged.

For PMD-mapped THP and hugetlb it's most certainly noise compared to the 
other activities when (un)mapping these large pages.

For PTE-mapped THP, it might be a bit bigger noise, although I doubt it 
is really significant (judging from my experience on managing 
PageAnonExclusive using set_bit/test_bit/clear_bit when (un)mapping anon 
pages).

As folio_add_file_rmap_range() indicates, for PTE-mapped THPs we should 
be batching where possible (and Ryan is working on some more rmap 
batching). There, managing the subpage mapcount dominates all other 
overhead significantly.

> 
> I had a feeling that there's some discussion behind the proposal of this
> patch, if that's the case it'll be great to attach the link in the commit
> log.

There were (mostly offline) discussions on how to sort out some other 
issues that PTE-mapped THP are facing, and how to eventually get rid of 
the subpage mapcounts (once consumer being _nr_pages_mapped as spelled 
out in the patch description). Having a proper total mapcount available 
was discussed as one building block.

I don't think I have anything of value to link that would make sense for 
the patch as is, as this patch is mostly independent from all that.

-- 
Cheers,

David / dhildenb