linux-kernel - Re: [PATCH v6 12/14] mm/gup: longterm pin migration cleaup

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Date:   Wed, 20 Jan 2021 09:17:57 -0500
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...e.com>,
        David Hildenbrand <david@...hat.com>,
        Oscar Salvador <osalvador@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Sasha Levin <sashal@...nel.org>,
        Tyler Hicks <tyhicks@...ux.microsoft.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v6 12/14] mm/gup: longterm pin migration cleaup

On Wed, Jan 20, 2021 at 8:19 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Tue, Jan 19, 2021 at 08:43:31PM -0500, Pavel Tatashin wrote:
> > When pages are longterm pinned, we must migrated them out of movable zone.
> > The function that migrates them has a hidden loop with goto. The loop is
> > to retry on isolation failures, and after successful migration.
> >
> > Make this code better by moving this loop to the caller.
> >
> > Signed-off-by: Pavel Tatashin <pasha.tatashin@...een.com>
> >  mm/gup.c | 88 +++++++++++++++++++++++---------------------------------
> >  1 file changed, 36 insertions(+), 52 deletions(-)
>
> This looks OK, it is better
>
> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
>
> I really dislike we always have to go over the page list twice in pin
> mode

I agree, I also dislike that we have to loop twice.

>
> The is_pinnable_page() and LRU isolation should really be done inside
> __get_user_pages_locked() as each page is added to the output list
>
> But that is more of a larger issue than this series

We could also think about adding some optimization flags, i.e. clients
could tell gup that the pinned memory content can be discarded
FOLL_DISCARD. That way we could always allocate new pages in the right
zones as we do with this series and free existing translations. No
migration check would be necessary with such a flag.

>
> Jason