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Date:   Fri, 18 Dec 2020 11:46:55 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Pavel Tatashin <pasha.tatashin@soleen.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, vbabka@suse.cz, david@redhat.com,
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Subject: Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures,
 honor failures
Message-ID: <20201218104655.GW32193@dhcp22.suse.cz>
References: <20201217185243.3288048-1-pasha.tatashin@soleen.com>
 <20201217185243.3288048-9-pasha.tatashin@soleen.com>
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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