linux-kernel - Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures, honor failures

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Message-ID: <20210115184006.GH4605@ziepe.ca>
Date:   Fri, 15 Jan 2021 14:40:06 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures,
 honor failures

On Fri, Jan 15, 2021 at 01:10:27PM -0500, Pavel Tatashin wrote:

> I've been thinking about this some more. Again, I am not sure this is
> a bug. I understand split_huge_pmd() may split the PMD size page into
> PTEs and leave the compound page intact. However, in order for pages[]
> to have non sequential addresses in compound page, those PTEs must
> also be migrated after split_huge_pmd(), 

Why focus on migrated? Anything could happen to those PTEs: they could
be COW'd, they could be mmap/munmap'd, etc.

Jason