On Mon, Dec 30, 2024 at 08:52:04PM +0100, David Hildenbrand wrote: > [...] > > I'm looking back at some of the discussions in v2 [1] and I'm still > > not clear on how memory fragmentation for non-movable pages differs > > from memory fragmentation from movable pages and whether one is worse > > than the other. Currently fuse uses movable temp pages (allocated with > > gfp flags GFP_NOFS | __GFP_HIGHMEM), and these can run into the same > > Why are they movable? Do you also specify __GFP_MOVABLE? > > If not, they are unmovable and are never allocated from > ZONE_MOVABLE/MIGRATE_CMA -- and usually only from MIGRATE_UNMOVBALE, to > group these unmovable pages. > Yes, these temp pages are non-movable. (Must be a typo in Joanne's email). [...] > > I assume not regarding fragmentation. > > > In general, I see two main issues: > > A) We are no longer waiting on writeback, even though we expect in sane > environments that writeback will happen and we it might be worthwhile to > just wait for writeback so we can migrate these folios. > > B) We allow turning movable pages to be unmovable, possibly forever/long > time, and there is no way to make them movable again (e.g., cancel > writeback). > > > I'm wondering if A) is actually a new issue introduced by this change. Can > folios with busy temp pages (writeback cleared on folio, but temp pages are > still around) be migrated? I will look into some details once I'm back from > vacation. > My suggestion is to just drop the patch related to A as it is not required for deadlock avoidance. For B, I think we need a long term solution which is usable by other filesystems as well.
