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

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Message-ID: <4cfb6fd5-f820-b56d-bbfe-13c92a5bf682@redhat.com>
Date:   Thu, 4 Aug 2022 09:40:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, jgg@...dia.com, minchan@...nel.org,
        linux-kernel@...r.kernel.org, jhubbard@...dia.com,
        pasha.tatashin@...een.com
Subject: Re: [PATCH v2] mm/gup.c: Simplify and fix
 check_and_migrate_movable_pages() return codes

On 04.08.22 02:12, Alistair Popple wrote:
> 
> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
>> On Tue,  2 Aug 2022 10:30:12 +1000 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().
>>>
>>> Fix this by simplifying the return codes such that zero indicates all
>>> pages were successfully pinned in the correct zone while errors indicate
>>> either pages were migrated and pinning should be retried or that
>>> migration has failed and therefore the pinning operation should fail.
>>>
>>> This fixes the indefinite looping on page isolation failure by failing
>>> the pin operation instead of retrying indefinitely.
>>>
>>
>> Are we able to identify a Fixes: for this?  Presumably something in the
>> series "Add MEMORY_DEVICE_COHERENT for coherent device memory mapping"?
> 
> It seems the infinite loop was desired behaviour so I will re-spin this
> as a pure clean-up.
> 

How can the infinite loop trigger when we allow longterm-pinning the
shared zeropage? (note: disallowing that for now was a bug)

-- 
Thanks,

David / dhildenb