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Date:   Fri, 11 Dec 2020 19:50:05 -0400
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     David Hildenbrand <david@redhat.com>
Cc:     Pavel Tatashin <pasha.tatashin@soleen.com>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Michal Hocko <mhocko@suse.com>,
        Oscar Salvador <osalvador@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Sasha Levin <sashal@kernel.org>,
        Tyler Hicks <tyhicks@linux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>, mike.kravetz@oracle.com,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Matthew Wilcox <willy@infradead.org>,
        David Rientjes <rientjes@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone
Message-ID: <20201211235005.GE5487@ziepe.ca>
References: <CA+CK2bCc9gk3Yy+ueaZVJs90MFE3fqukLsdb5R2kTUH4tWRbkA@mail.gmail.com>
 <447A41F3-EB94-4DA4-8B98-038B127774A5@redhat.com>
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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