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

[Michal Hocko]

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