linux-kernel - Re: [PATCH v2 2/2] mm/hugetlb: refactor subpage recording

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Message-ID: <68570840-78E9-4B32-BE0B-B6DC60479DB0@nvidia.com>
Date:   Thu, 11 Feb 2021 15:47:46 -0500
From:   Zi Yan <ziy@...dia.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Joao Martins <joao.m.martins@...cle.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 2/2] mm/hugetlb: refactor subpage recording

On 28 Jan 2021, at 16:53, Mike Kravetz wrote:

> On 1/28/21 10:26 AM, Joao Martins wrote:
>> For a given hugepage backing a VA, there's a rather ineficient
>> loop which is solely responsible for storing subpages in GUP
>> @pages/@...s array. For each subpage we check whether it's within
>> range or size of @pages and keep increment @pfn_offset and a couple
>> other variables per subpage iteration.
>>
>> Simplify this logic and minimize the cost of each iteration to just
>> store the output page/vma. Instead of incrementing number of @refs
>> iteratively, we do it through pre-calculation of @refs and only
>> with a tight loop for storing pinned subpages/vmas.
>>
>> Additionally, retain existing behaviour with using mem_map_offset()
>> when recording the subpages for configurations that don't have a
>> contiguous mem_map.
>>
>> pinning consequently improves bringing us close to
>> {pin,get}_user_pages_fast:
>>
>>   - 16G with 1G huge page size
>>   gup_test -f /mnt/huge/file -m 16384 -r 30 -L -S -n 512 -w
>>
>> PIN_LONGTERM_BENCHMARK: ~12.8k us -> ~5.8k us
>> PIN_FAST_BENCHMARK: ~3.7k us
>>
>> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
>> ---
>>  mm/hugetlb.c | 49 ++++++++++++++++++++++++++++---------------------
>>  1 file changed, 28 insertions(+), 21 deletions(-)
>
> Thanks for updating this.
>
> Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>
>
> I think there still is an open general question about whether we can always
> assume page structs are contiguous for really big pages.  That is outside

I do not think page structs need to be contiguous, but PFNs within a big page
need to be contiguous, at least based on existing code like mem_map_offset() we have.
The assumption seems valid according to the existing big page allocation methods,
which use alloc_contig_pages() at the end of the day. alloc_contig_pages()
calls pfn_range_valid_contig() to make sure all PFNs are contiguous.
On the other hand, the buddy allocator only merges contiguous PFNs, so there
will be no problem even if someone configures the buddy allocator to allocate
gigantic pages.

Unless someone comes up with some fancy way of making page allocations from
contiguous page structs in SPARSEMEM_VMEMMAP case, where non-contiguous
PFNs with contiguous page structs are possible, or out of any adjacent
pages in !SPARSEMEM_VMEMMAP case, where non-contiguous page structs
and non-contiguous PFNs are possible, we should be good.

—
Best Regards,
Yan Zi

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