linux-kernel - Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page during isolation

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Date:   Wed, 3 Feb 2021 10:53:13 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Joao Martins <joao.m.martins@...cle.com>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...e.com,
        david@...hat.com, osalvador@...e.de, dan.j.williams@...el.com,
        sashal@...nel.org, tyhicks@...ux.microsoft.com,
        iamjoonsoo.kim@....com, mike.kravetz@...cle.com,
        rostedt@...dmis.org, mingo@...hat.com, peterz@...radead.org,
        mgorman@...e.de, willy@...radead.org, rientjes@...gle.com,
        jhubbard@...dia.com, linux-doc@...r.kernel.org,
        ira.weiny@...el.com, linux-kselftest@...r.kernel.org,
        jmorris@...ei.org
Subject: Re: [PATCH v8 02/14] mm/gup: check every subpage of a compound page
 during isolation

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.

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

> 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

Jason