Re: [PATCH v3 5/6] mm/gup: migrate pinned pages out of movable zone

[Jason Gunthorpe]

On Fri, Dec 11, 2020 at 10:53:00PM +0100, David Hildenbrand wrote:

> > When check_and_migrate_movable_pages() is called, the pages are
> > already pinned. If some of those pages are in movable zone, and we
> > fail to migrate or isolate them what should we do: proceed, and
> > keep it as exception of when movable zone can actually have pinned
> > pages or unpin all pages in the array, and return an error, or
> > unpin only pages in movable zone, and return an error?
> 
> I guess revert what we did (unpin) and return an error. The
> interesting question is what can make migration/isolation fail
> 
> a) out of memory: smells like a zone setup issue. Failures are acceptable I guess.

Out of memory is reasonable..
 
> b) short term pinnings: process dying - not relevant I guess. Other cases? (Fork?)

Concurrent with non-longterm GUP users are less reasonable, fork is
not reasonable, etc..

Racing with another GUP in another thread is also not reasonable, so
failing to isolate can't be a failure

Jasnon