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

[Aneesh Kumar K.V]

On 12/20/18 11:18 AM, Alexey Kardashevskiy wrote:
> On 20/12/2018 16:22, Aneesh Kumar K.V wrote:
> > 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>
> >
> > .....
> > > > +ÂÂÂÂÂÂÂÂ * We did migrate all the pages, Try to get the page
> > > > references again
> > > > +ÂÂÂÂÂÂÂÂ * migrating any new CMA pages which we failed to isolate
> > > > earlier.
> > > > +ÂÂÂÂÂÂÂÂ */
> > > > +ÂÂÂÂÂÂÂ drain_allow = true;
> > > > +ÂÂÂÂÂÂÂ 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.
>
>
> Then migrate_allow should be set to "false" regardless what
> migrate_pages() returned and then I am totally missing the point of this
> goto and going through the loop again even when we know for sure it
> won't do literally anything but checking is_migrate_cma_page() even
> though we know pages won't be allocated from CMA.

Because we might have failed to isolate all the pages in the first attempt.

-aneesh