Re: [PATCH v6 4/5] mm/migrate: skip migrating folios under writeback with AS_WRITEBACK_INDETERMINATE mappings

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Jan 06, 2025 at 11:19:42AM +0100, Miklos Szeredi wrote:
> On Fri, 3 Jan 2025 at 21:31, David Hildenbrand <david@redhat.com> wrote:
> > In any case, having movable pages be turned unmovable due to persistent
> > writaback is something that must be fixed, not worked around. Likely a
> > good topic for LSF/MM.
> 
> Yes, this seems a good cross fs-mm topic.
> 
> So the issue discussed here is that movable pages used for fuse
> page-cache cause a problems when memory needs to be compacted. The
> problem is either that
> 
>  - the page is skipped, leaving the physical memory block unmovable
> 
>  - the compaction is blocked for an unbounded time
> 
> While the new AS_WRITEBACK_INDETERMINATE could potentially make things
> worse, the same thing happens on readahead, since the new page can be
> locked for an indeterminate amount of time, which can also block
> compaction, right?

Yes locked pages are unmovable. How much of these locked pages/folios
can be caused by untrusted fuse server?

> 
> What about explicitly opting fuse cache pages out of compaction by
> allocating them form ZONE_UNMOVABLE?

This can be done but it will change the memory condition of the
users/workloads/systems where page cache is the majority of the memory
(i.e. majority of memory will be unmovable) and when such systems are
overcommitted, weird corner cases will arise (failing high order
allocations, long term fragmentation etc). In addition the memory
behind CXL will become unusable for fuse folios.

IMHO the transient unmovable state of fuse folios due to writeback is
not an issue if we can show that untrusted fuse server can not cause
unlimited folios under writeback for arbitrary long time.