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

From: Pavel Tatashin
Date: Wed Jan 20 2021 - 09:28:33 EST

On Wed, Jan 20, 2021 at 8:19 AM Jason Gunthorpe <jgg@xxxxxxxx> 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@xxxxxxxxxx>
> > mm/gup.c | 88 +++++++++++++++++++++++---------------------------------
> > 1 file changed, 36 insertions(+), 52 deletions(-)
> This looks OK, it is better
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> 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