On 11/24/21 09:57, David Hildenbrand wrote:

On 24.11.21 00:59, Jason Gunthorpe wrote:

quoted

quoted

Similarly for io-uring we could be migrating pages to be pinned so that
the end up consolidated close together, and prevent pathologic
situations like in David's reproducer. 

It is an interesting idea to have GUP do some kind of THP preserving
migration.

Unfortunately it will only be a band aid AFAIU. I can rewrite my
reproducer fairly easily to pin the whole 2M range first, pin a second
time only a single page, and then unpin the 2M range, resulting in the
very same way to block THP. (I can block some THP less because I always
need the possibility to memlock 2M first, though).

Hm I see, then we could also condsider making it possible to migrate the
pinned pages - of course io-uring would have to be cooperative here,
similarly to anything that supports PageMovable. Mlocked pages can also be
migrated (I think there's some sysctl to prevent that, if the guarantee of
not even a minor pagefault is considered more important).