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

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Date:   Fri, 18 Dec 2020 11:46:55 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, 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,
        jgg@...pe.ca, 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
Subject: Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures,
 honor failures

On Thu 17-12-20 13:52:41, Pavel Tatashin wrote:
[...]
> +#define PINNABLE_MIGRATE_MAX	10
> +#define PINNABLE_ISOLATE_MAX	100

Why would we need to limit the isolation retries. Those should always be
temporary failure unless I am missing something. I am not sure about the
PINNABLE_MIGRATE_MAX either. Why do we want to limit that? migrate_pages
already implements its retry logic why do you want to count retries on
top of that? I do agree that the existing logic is suboptimal because
the migration failure might be ephemeral or permanent but that should be
IMHO addressed at migrate_pages (resp. unmap_and_move) and simply report
failures that are permanent - e.g. any potential pre-existing long term
pin - if that is possible at all. If not what would cause permanent
migration failure? OOM?
-- 
Michal Hocko
SUSE Labs