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Date:   Thu, 10 Aug 2023 13:14:03 +0200
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From:   David Hildenbrand <david@redhat.com>
To:     Ryan Roberts <ryan.roberts@arm.com>, linux-kernel@vger.kernel.org
Cc:     linux-mm@kvack.org, linux-doc@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Hugh Dickins <hughd@google.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yang Shi <shy828301@gmail.com>, Zi Yan <ziy@nvidia.com>
References: <20230809083256.699513-1-david@redhat.com>
 <181fcc79-b1c6-412f-9ca1-d1f21ef33e32@arm.com>
 <60b5b2a2-1d1d-661c-d61e-855178fff44d@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH mm-unstable v1] mm: add a total mapcount for large folios
In-Reply-To: <60b5b2a2-1d1d-661c-d61e-855178fff44d@redhat.com>
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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@linux-foundation.org>
>>> Cc: Jonathan Corbet <corbet@lwn.net>
>>> Cc: Mike Kravetz <mike.kravetz@oracle.com>
>>> Cc: Hugh Dickins <hughd@google.com>
>>> Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>>> Cc: Yin Fengwei <fengwei.yin@intel.com>
>>> Cc: Yang Shi <shy828301@gmail.com>
>>> Cc: Zi Yan <ziy@nvidia.com>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>
>> Other than the nits and query on zeroing _total_mapcount below, LGTM. If zeroing
>> is correct:
>>
>> Reviewed-by: Ryan Roberts <ryan.roberts@arm.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 :)

-- 
Cheers,

David / dhildenb