Re: [PATCH v2] mm/gup.c: Simplify and fix check_and_migrate_movable_pages() return codes

[Alistair Popple]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Tue,  2 Aug 2022 10:30:12 +1000 Alistair Popple <apopple@xxxxxxxxxx> 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.