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

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Message-ID: <CA+CK2bAok_0Q+AovxVYqtsObVbbxr+ZZcTgP76uxEvDy-uJWsQ@mail.gmail.com>
Date:   Fri, 11 Dec 2020 18:00:33 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        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

> I guess revert what we did (unpin) and return an error. The interesting question is what can make migration/isolation fail

OK. I will make the necessary changes. Let's handle errors properly.
Whatever the cause for the error, we will know it when it happens, and
when error is returned. I think I will add a 10-time retry instead of
the infinite retry that we currently have. The 10-times retry we
currently have during the hot-remove path.

>
> a) out of memory: smells like a zone setup issue. Failures are acceptable I guess.
>
> b) short term pinnings: process dying - not relevant I guess. Other cases? (Fork?)
>
> c) ?
>
> Once we clarified that, we actually know how likely it will be to return an error (and making vfio pinnings fail etc).