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

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Date:   Tue, 2 Aug 2022 17:12:15 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, jgg@...dia.com, minchan@...nel.org,
        linux-kernel@...r.kernel.org, jhubbard@...dia.com,
        pasha.tatashin@...een.com, david@...hat.com
Subject: Re: [PATCH v2] mm/gup.c: Simplify and fix
 check_and_migrate_movable_pages() return codes

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"?