Re: [PATCH V5 1/3] mm: Add get_user_pages_cma_migrate

[Aneesh Kumar K.V]

On 12/20/18 9:49 AM, Alexey Kardashevskiy wrote:
> On 19/12/2018 14:40, Aneesh Kumar K.V wrote:
> > This helper does a get_user_pages_fast and if it find pages in the CMA area
> > it will try to migrate them before taking page reference. This makes sure that
> > we don't keep non-movable pages (due to page reference count) in the CMA area.
> > Not able to move pages out of CMA area result in CMA allocation failures.
> >
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>

.....
> > +		 * We did migrate all the pages, Try to get the page references again
> > +		 * migrating any new CMA pages which we failed to isolate earlier.
> > +		 */
> > +		drain_allow = true;
> > +		goto get_user_again;
>
>
> So it is possible to have pages pinned, then successfully migrated
> (migrate_pages() returned 0), then pinned again, then some pages may end
> up in CMA again and migrate again and nothing seems to prevent this loop
> from being endless. What do I miss?

pages used as target page for migration won't be allocated from CMA region.

-aneesh