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

From: Aneesh Kumar K.V
Date: Thu Dec 20 2018 - 00:55:49 EST

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

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
+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.