On Fri, 2025-01-10 at 21:20 +0100, David Hildenbrand wrote: > On 10.01.25 21:16, Jeff Layton wrote: > > On Tue, 2025-01-07 at 09:34 +0100, David Hildenbrand wrote: > > > 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. > > > > > > > I imagine that you can just open a file and start writing to it, pull > > the plug on the NFS server, and then issue a fsync or something to > > ensure some writeback occurs. > > Yes, that's the plan, thanks! > > > > > Any dirty pagecache folios should be stuck in writeback at that point. > > The NFS client is also very patient about waiting for the server to > > come back, so it should stay that way indefinitely. > > Yes, however the default timeout for UDP is fairly small (for TCP > certainly much longer). So one thing I'd like to understand what that > "cancel writeback -> redirty folio" on timeout does, and when it > actually triggers with TCP vs UDP timeouts. > The lifetime of the pagecache pages is not at all related to the socket lifetimes. IOW, the client can completely lose the connection to the server and the page will just stay dirty until the connection can be reestablished and the server responds. The exception here is if you mount with "-o soft" in which case, an RPC request will time out with an error after a major RPC timeout (usually after a minute or so). See nfs(5) for the gory details of timeouts and retransmission. The default is "-o hard" since that's necessary for data-integrity in the face of spotty network connections. Once a soft mount has a writeback RPC time out, the folio is marked clean and a writeback error is set on the mapping, so that fsync() will return an error. -- Jeff Layton <jlayton@xxxxxxxxxx>
