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

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Date:   Fri, 11 Aug 2023 00:27:13 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>, Peter Xu <peterx@...hat.com>
Cc:     Ryan Roberts <ryan.roberts@....com>, 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>,
        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 23:48, Matthew Wilcox wrote:
> On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote:
>> AFAICS if that patch was all correct (while I'm not yet sure..), you can
>> actually fit your new total mapcount field into page 1 so even avoid the
>> extra cacheline access.  You can have a look: the trick is refcount for
>> tail page 1 is still seems to be free on 32 bits (if that was your worry
>> before).  Then it'll be very nice if to keep Hugh's counter all in tail 1.
> 
> No, refcount must be 0 on all tail pages.  We rely on this in many places
> in the MM.

Very right.

One could theoretically

1) Move the compound/entire mapcount to page[2]
2) Make hugetlb stop using the entire mapcount and only the total
    mapcount.
3) Then leave total_mapcount and nr_pages_mapped in page[1]
4) Make page_mapcount() use total_mapcount for hugetlb.

When (un)mapping a PMD-mapped THP, we would go to page[2]. Otherwise, 
only page[1].

The entire mapcount, similarly to nr_pages_mapped, primarily serves to 
get memory stats right; well, and to implement page_mapcount() for THP.

But I'm not 100% sure yet if the overhead from having nr_pages_mapped in 
page[2] is significant enough at this point.

-- 
Cheers,

David / dhildenb