On 06.01.25 19:17, Shakeel Butt wrote:
On Mon, Jan 06, 2025 at 11:19:42AM +0100, Miklos Szeredi wrote:
On Fri, 3 Jan 2025 at 21:31, David Hildenbrand <david@xxxxxxxxxx> 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, as memory hotplug + virtio-mem maintainer my bigger concern is
these pages residing in ZONE_MOVABLE / MIGRATE_CMA areas where there
*must not be unmovable pages ever*. Not triggered by an untrusted
source, not triggered by an trusted source.
It's a violation of core-mm principles.
Even if we have a timeout of 60s, making things like alloc_contig_page()
wait for that long on writeback is broken and needs to be fixed.
And the fix is not to skip these pages, that's a workaround.
I'm hoping I can find an easy way to trigger this also with NFS.
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.
Yes.
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.
See above, I disagree.
--
Cheers,
David / dhildenb
