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

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Date:   Fri, 11 Aug 2023 00:16:52 +0200
From:   David Hildenbrand <david@...hat.com>
To:     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>,
        "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

>> Okay, so your speculation right now is:
>>
>> 1) The change in cacheline might be problematic.
>>
>> 2) The additional atomic operation might be problematic.
>>
>>> then measure the split (by e.g. mprotect() at offset 1M on a 4K?) time it
>>> takes before/after this patch.
>>
>> I can certainly try getting some numbers on that. If you're aware of other
>> micro-benchmarks that would likely notice slower pte-mapping of THPs, please
>> let me know.
> 
> Thanks.

If I effectively only measure the real PTE->PMD remapping (only measure 
the for loop that mprotects() one 4k page inside each of 512 THPs ) 
without any of the mmap+populate+munmap, I can certainly measure a real 
difference.

I briefly looked at some perf data across the overall benchmark runtime.

For page_remove_rmap(), the new atomic_dec() doesn't seem to be 
significant. Data indicates that it's significantly less relevant than a 
later atomic_add_negative().

For page_add_anon_rmap(), it's a bit fuzzy. Definitely, the 
atomic_inc_return_relaxed(mapped) seems to stick out, but I cannot rule 
out that the atomic_add() also plays a role.

The PTE->PMD remapping effectively does (__split_huge_pmd_locked())

for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
	...
	page_add_anon_rmap(page + i, vma, addr, RMAP_NONE);
	...
}
...
page_remove_rmap(page, vma, true);

Inside that loop we're repeatedly accessing the total_mapcount and 
_nr_pages_mapped. So my best guess would have been that both are already 
hot in the cache.

RMAP batching certainly sounds like a good idea for 
__split_huge_pmd_locked(), independent of this patch.

What would probably also interesting is observing happens when we unmap 
a single PTE of a THP and we cannot batch, to see if the 
page_remove_rmap() matters in the bigger scale.

I'll do some more digging tomorrow to clarify some details. Running some 
kernel compile tests with thp=always at least didn't reveal any 
surprises so far.

-- 
Cheers,

David / dhildenb