Message-ID: <fcef6eac-8bb3-45dc-b0a1-a5bd65580ff7@arm.com>
Date:   Thu, 10 Aug 2023 12:35:50 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     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>,
        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 10/08/2023 12:32, David Hildenbrand wrote:
> On 10.08.23 13:27, David Hildenbrand wrote:
>> On 10.08.23 13:14, David Hildenbrand wrote:
>>> On 09.08.23 21:17, David Hildenbrand wrote:
>>>> On 09.08.23 21:07, Ryan Roberts wrote:
>>>>> On 09/08/2023 09:32, 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().
>>>>>>
>>>>>> 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.
>>>>>>
>>>>>> Make room for the total mapcount in page[1] of the folio by moving
>>>>>> _nr_pages_mapped to page[2] of the folio: it is not applicable to hugetlb
>>>>>> -- and with the total mapcount in place probably also not desirable even
>>>>>> if PMD-mappable hugetlb pages could get PTE-mapped at some point -- so we
>>>>>> can overlay the hugetlb fields.
>>>>>>
>>>>>> Note that we currently don't expect any order-1 compound pages / THP in
>>>>>> rmap code, and that such support is not planned. If ever desired, we could
>>>>>> move the compound mapcount to another page, because it only applies to
>>>>>> PMD-mappable folios that are definitely larger. Let's avoid consuming
>>>>>> more space elsewhere for now -- we might need more space for a different
>>>>>> purpose in some subpages soon.
>>>>>>
>>>>>> Maintain the total mapcount also for hugetlb pages. Use the total mapcount
>>>>>> to implement folio_mapcount(), total_mapcount(), folio_mapped() and
>>>>>> page_mapped().
>>>>>>
>>>>>> We can now get rid of folio_total_mapcount() and
>>>>>> folio_large_is_mapped(), by just inlining reading of the total mapcount.
>>>>>>
>>>>>> _nr_pages_mapped is now only used in rmap code, so not accidentially
>>>>>> externally where it might be used on arbitrary order-1 pages. The remaining
>>>>>> usage is:
>>>>>>
>>>>>> (1) Detect how to adjust stats: NR_ANON_MAPPED and NR_FILE_MAPPED
>>>>>>      -> If we would account the total folio as mapped when mapping a
>>>>>>         page (based on the total mapcount), we could remove that usage.
>>>>>>
>>>>>> (2) Detect when to add a folio to the deferred split queue
>>>>>>      -> If we would apply a different heuristic, or scan using the rmap on
>>>>>>         the memory reclaim path for partially mapped anon folios to
>>>>>>         split them, we could remove that usage as well.
>>>>>>
>>>>>> So maybe, we can simplify things in the future and remove
>>>>>> _nr_pages_mapped. For now, leave these things as they are, they need more
>>>>>> thought. Hugh really did a nice job implementing that precise tracking
>>>>>> after all.
>>>>>>
>>>>>> Note: Not adding order-1 sanity checks to the file_rmap functions for
>>>>>>           now. For anon pages, they are certainly not required right now.
>>>>>>
>>>>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
>>>>>> Cc: Jonathan Corbet <corbet@....net>
>>>>>> Cc: Mike Kravetz <mike.kravetz@...cle.com>
>>>>>> Cc: Hugh Dickins <hughd@...gle.com>
>>>>>> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
>>>>>> Cc: Ryan Roberts <ryan.roberts@....com>
>>>>>> Cc: Yin Fengwei <fengwei.yin@...el.com>
>>>>>> Cc: Yang Shi <shy828301@...il.com>
>>>>>> Cc: Zi Yan <ziy@...dia.com>
>>>>>> Signed-off-by: David Hildenbrand <david@...hat.com>
>>>>>
>>>>> Other than the nits and query on zeroing _total_mapcount below, LGTM. If
>>>>> zeroing
>>>>> is correct:
>>>>>
>>>>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>>>>
>>>> Thanks for the review!
>>>>
>>>> [...]
>>>>
>>>>>>           static inline int total_mapcount(struct page *page)
>>>>>
>>>>> nit: couldn't total_mapcount() just be implemented as a wrapper around
>>>>> folio_mapcount()? What's the benefit of PageCompound() check instead of just
>>>>> getting the folio and checking if it's large? i.e:
>>>>
>>>> Good point, let me take a look tomorrow if the compiler can optimize in
>>>> both cases equally well.
>>>
>>> I checked by adjusting total_mapcount():
>>>
>>> Before:
>>>
>>>            if (PageTransHuge(page) && total_mapcount(page) > 1)
>>> ffffffff81411931:       4c 89 e7                mov    %r12,%rdi
>>> ffffffff81411934:       e8 f7 b1 ff ff          call   ffffffff8140cb30
>>> <PageTransHuge>
>>> ffffffff81411939:       85 c0                   test   %eax,%eax
>>> ffffffff8141193b:       74 29                   je     ffffffff81411966
>>> <migrate_misplaced_page+0x166>
>>> ffffffff8141193d:       49 8b 04 24             mov    (%r12),%rax
>>>            return test_bit(PG_head, &page->flags) ||
>>> ffffffff81411941:       a9 00 00 01 00          test   $0x10000,%eax
>>> ffffffff81411946:       0f 85 1f 01 00 00       jne    ffffffff81411a6b
>>> <migrate_misplaced_page+0x26b>
>>>                   READ_ONCE(page->compound_head) & 1;
>>> ffffffff8141194c:       49 8b 44 24 08          mov    0x8(%r12),%rax
>>>            return test_bit(PG_head, &page->flags) ||
>>> ffffffff81411951:       a8 01                   test   $0x1,%al
>>> ffffffff81411953:       0f 85 12 01 00 00       jne    ffffffff81411a6b
>>> <migrate_misplaced_page+0x26b>
>>> ffffffff81411959:       41 8b 44 24 30          mov    0x30(%r12),%eax
>>>                    return atomic_read(&page->_mapcount) + 1;
>>> ffffffff8141195e:       83 c0 01                add    $0x1,%eax
>>> ffffffff81411961:       83 f8 01                cmp    $0x1,%eax
>>> ffffffff81411964:       7f 77                   jg     ffffffff814119dd
>>> <migrate_misplaced_page+0x1dd>
>>>
>>> So a total of 10 instructions after handling the mov/call/test/je for
>>> PageTransHuge().
>>>
>>> After:
>>>
>>>            if (PageTransHuge(page) && total_mapcount(page) > 1)
>>> ffffffff81411931:       4c 89 e7                mov    %r12,%rdi
>>> ffffffff81411934:       e8 f7 b1 ff ff          call   ffffffff8140cb30
>>> <PageTransHuge>
>>> ffffffff81411939:       85 c0                   test   %eax,%eax
>>> ffffffff8141193b:       74 2f                   je     ffffffff8141196c
>>> <migrate_misplaced_page+0x16c>
>>>            unsigned long head = READ_ONCE(page->compound_head);
>>> ffffffff8141193d:       49 8b 44 24 08          mov    0x8(%r12),%rax
>>>            if (unlikely(head & 1))
>>> ffffffff81411942:       a8 01                   test   $0x1,%al
>>> ffffffff81411944:       0f 85 fc 05 00 00       jne    ffffffff81411f46
>>> <migrate_misplaced_page+0x746>
>>> ffffffff8141194a:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
>>>                    return page;
>>> ffffffff8141194f:       4c 89 e0                mov    %r12,%rax
>>> ffffffff81411952:       48 8b 10                mov    (%rax),%rdx
>>>            if (likely(!folio_test_large(folio)))
>>> ffffffff81411955:       f7 c2 00 00 01 00       test   $0x10000,%edx
>>> ffffffff8141195b:       0f 85 da 05 00 00       jne    ffffffff81411f3b
>>> <migrate_misplaced_page+0x73b>
>>> ffffffff81411961:       8b 40 30                mov    0x30(%rax),%eax
>>>                    return atomic_read(&folio->_mapcount) + 1;
>>> ffffffff81411964:       83 c0 01                add    $0x1,%eax
>>> ffffffff81411967:       83 f8 01                cmp    $0x1,%eax
>>> ffffffff8141196a:       7f 77                   jg     ffffffff814119e3
>>> <migrate_misplaced_page+0x1e3>
>>>
>>> So a total of 11 (+ 1 NOP) instructions after handling the mov/call/test/je
>>> for PageTransHuge().
>>>
>>> Essentially one more MOV instruction.
>>>
>>> I guess nobody really cares :)
>>>
>>
>> Also, let's simply do:
>>
>> static inline bool folio_mapped(struct folio *folio)
>> {
>>     folio_mapcount(folio) > 1;
> 
>> 0, obviously :)
> 

;-) All sounds good to me!

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