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Subject: Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page
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To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Pavel Tatashin <pasha.tatashin@soleen.com>,
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References: <20210125194751.1275316-1-pasha.tatashin@soleen.com>
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From:   Joao Martins <joao.m.martins@oracle.com>
Message-ID: <f82dca38-52bd-aa27-2d27-b5a67f5284f5@oracle.com>
Date:   Wed, 3 Feb 2021 16:13:21 +0000
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On 2/3/21 2:53 PM, Jason Gunthorpe wrote:
> On Wed, Feb 03, 2021 at 01:22:18PM +0000, Joao Martins wrote:
> 
>> With this, longterm gup will 'regress' for hugetlbfs e.g. from ~6k -> ~32k usecs when
>> pinning a 16G hugetlb file.
> 
> Yes, but correctness demands it.
> 
Hence the quotes around the word regress.

But still, we are adding unnecessary overhead (previously nonexistent) to compound pages
even those where the issue doesn't exist (hugetlb).

> The solution is to track these pages as we discover them so we know if
> a PMD/PUD points and can directly skip the duplicated work
> 
That looks better. It would require moving check_and_migrate_movable_pages() elsewhere,
and/or possibly reworking the entire series?

>> Splitting can only occur on THP right? If so, perhaps we could
>> retain the @step increment for compound pages but when
>> !is_transparent_hugepage(head) or just PageHuge(head) like:
> 
> Honestly I'd rather see it fixed properly which will give even bigger
> performance gains - avoiding the entire rescan of the page list will
> be a win
> 

If check_and_migrate_movable_pages() is meant to migrate unpinned pages, then rather than
pinning+unpinning+moving, perhaps it would be called in __get_user_pages() place where we
are walking page tables and know if it's a PUD/PMD and can skip all the subpages and just
record and migrate those instead. Was that your thinking?

	Joao