linux-kernel - Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone

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Message-ID: <20201211235005.GE5487@ziepe.ca>
Date:   Fri, 11 Dec 2020 19:50:05 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     David Hildenbrand <david@...hat.com>
Cc:     Pavel Tatashin <pasha.tatashin@...een.com>,
        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>,
        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>
Subject: Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone

On Fri, Dec 11, 2020 at 10:53:00PM +0100, David Hildenbrand wrote:

> > When check_and_migrate_movable_pages() is called, the pages are
> > already pinned. If some of those pages are in movable zone, and we
> > fail to migrate or isolate them what should we do: proceed, and
> > keep it as exception of when movable zone can actually have pinned
> > pages or unpin all pages in the array, and return an error, or
> > unpin only pages in movable zone, and return an error?
> 
> I guess revert what we did (unpin) and return an error. The
> interesting question is what can make migration/isolation fail
> 
> a) out of memory: smells like a zone setup issue. Failures are acceptable I guess.

Out of memory is reasonable..
 
> b) short term pinnings: process dying - not relevant I guess. Other cases? (Fork?)

Concurrent with non-longterm GUP users are less reasonable, fork is
not reasonable, etc..

Racing with another GUP in another thread is also not reasonable, so
failing to isolate can't be a failure

Jasnon