linux-kernel - Re: [PATCH v2] mm/gup.c: Simplify and fix check_and_migrate_movable

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Message-ID: <CA+CK2bCxuSs8aDre7B7cr=B1FpD+d1kJL_1uCU1qtNKErpUSSQ@mail.gmail.com>
Date:   Tue, 2 Aug 2022 17:36:41 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm <linux-mm@...ck.org>, jgg@...dia.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v2] mm/gup.c: Simplify and fix check_and_migrate_movable_pages()
 return codes

On Mon, Aug 1, 2022 at 8:32 PM Alistair Popple <apopple@...dia.com> wrote:
>
> When pinning pages with FOLL_LONGTERM check_and_migrate_movable_pages()
> is called to migrate pages out of zones which should not contain any
> longterm pinned pages.
>
> When migration succeeds all pages will have been unpinned so pinning
> needs to be retried. This is indicated by returning zero. When all pages
> are in the correct zone the number of pinned pages is returned.
>
> However migration can also fail, in which case pages are unpinned and
> -ENOMEM is returned. However if the failure was due to not being unable
> to isolate a page zero is returned. This leads to indefinite looping in
> __gup_longterm_locked().

Hi Alistair,

During prohibiting pinning movable zone development, there was a
discussion where we figured that isolation errors should be transient
[1]. What isolation errors are you seeing that lead to infinite loop?
Why do they happen?

Pasha

[1] https://lore.kernel.org/linux-mm/20201218104655.GW32193@dhcp22.suse.cz