On Wed, Oct 30, 2024 at 9:21 AM Bernd Schubert <bernd.schubert@xxxxxxxxxxx> wrote: > > On 10/30/24 17:04, Joanne Koong wrote: > > On Wed, Oct 30, 2024 at 2:32 AM Bernd Schubert > > <bernd.schubert@xxxxxxxxxxx> wrote: > >> > >> On 10/28/24 22:58, Joanne Koong wrote: > >>> On Fri, Oct 25, 2024 at 3:40 PM Joanne Koong <joannelkoong@xxxxxxxxx> wrote: > >>>> > >>>>> Same here, I need to look some more into the compaction / page > >>>>> migration paths. I'm planning to do this early next week and will > >>>>> report back with what I find. > >>>>> > >>>> > >>>> These are my notes so far: > >>>> > >>>> * We hit the folio_wait_writeback() path when callers call > >>>> migrate_pages() with mode MIGRATE_SYNC > >>>> ... -> migrate_pages() -> migrate_pages_sync() -> > >>>> migrate_pages_batch() -> migrate_folio_unmap() -> > >>>> folio_wait_writeback() > >>>> > >>>> * These are the places where we call migrate_pages(): > >>>> 1) demote_folio_list() > >>>> Can ignore this. It calls migrate_pages() in MIGRATE_ASYNC mode > >>>> > >>>> 2) __damon_pa_migrate_folio_list() > >>>> Can ignore this. It calls migrate_pages() in MIGRATE_ASYNC mode > >>>> > >>>> 3) migrate_misplaced_folio() > >>>> Can ignore this. It calls migrate_pages() in MIGRATE_ASYNC mode > >>>> > >>>> 4) do_move_pages_to_node() > >>>> Can ignore this. This calls migrate_pages() in MIGRATE_SYNC mode but > >>>> this path is only invoked by the move_pages() syscall. It's fine to > >>>> wait on writeback for the move_pages() syscall since the user would > >>>> have to deliberately invoke this on the fuse server for this to apply > >>>> to the server's fuse folios > >>>> > >>>> 5) migrate_to_node() > >>>> Can ignore this for the same reason as in 4. This path is only invoked > >>>> by the migrate_pages() syscall. > >>>> > >>>> 6) do_mbind() > >>>> Can ignore this for the same reason as 4 and 5. This path is only > >>>> invoked by the mbind() syscall. > >>>> > >>>> 7) soft_offline_in_use_page() > >>>> Can skip soft offlining fuse folios (eg folios with the > >>>> AS_NO_WRITEBACK_WAIT mapping flag set). > >>>> The path for this is soft_offline_page() -> soft_offline_in_use_page() > >>>> -> migrate_pages(). soft_offline_page() only invokes this for in-use > >>>> pages in a well-defined state (see ret value of get_hwpoison_page()). > >>>> My understanding of soft offlining pages is that it's a mitigation > >>>> strategy for handling pages that are experiencing errors but are not > >>>> yet completely unusable, and its main purpose is to prevent future > >>>> issues. It seems fine to skip this for fuse folios. > >>>> > >>>> 8) do_migrate_range() > >>>> 9) compact_zone() > >>>> 10) migrate_longterm_unpinnable_folios() > >>>> 11) __alloc_contig_migrate_range() > >>>> > >>>> 8 to 11 needs more investigation / thinking about. I don't see a good > >>>> way around these tbh. I think we have to operate under the assumption > >>>> that the fuse server running is malicious or benevolently but > >>>> incorrectly written and could possibly never complete writeback. So we > >>>> definitely can't wait on these but it also doesn't seem like we can > >>>> skip waiting on these, especially for the case where the server uses > >>>> spliced pages, nor does it seem like we can just fail these with > >>>> -EBUSY or something. > >> > >> I see some code paths with -EAGAIN in migration. Could you explain why > >> we can't just fail migration for fuse write-back pages? > >> > > Hi Joanne, > > thanks a lot for your quick reply (especially as my reviews come in very > late). > Thanks for your comments/reviews, Bernd! I always appreciate them. > > > > My understanding (and please correct me here Shakeel if I'm wrong) is > > that this could block system optimizations, especially since if an > > unprivileged malicious fuse server never replies to the writeback > > request, then this completely stalls progress. In the best case > > scenario, -EAGAIN could be used because the server might just be slow > > in serving the writeback, but I think we need to also account for > > servers that never complete the writeback. For > > __alloc_contig_migrate_range() for example, my understanding is that > > this is used to migrate pages so that there are more physically > > contiguous ranges of memory freed up. If fuse writeback blocks that, > > then that hurts system health overall. > > Hmm, I wonder what is worse - tmp page copies or missing compaction. > Especially if we expect a low range of in-writeback pages/folios. > One could argue that an evil user might spawn many fuse server > processes to work around the default low fuse write-back limits, but > does that make any difference with tmp pages? And these cannot be > compacted either? My understanding (and Shakeel please jump in here if this isn't right) is that tmp pages can be migrated/compacted. I think it's only pages marked as under writeback that are considered to be non-movable. > > And with timeouts that would be so far totally uncritical, I > think. > > > You also mentioned > > > especially for the case where the server uses spliced pages > > could you provide more details for that? > For the page migration / compaction paths, I don't think we can do the workaround we could do for sync where we skip waiting on writeback for fuse folios and continue on with the operation, because the migration / compaction paths operate on the pages. For the splice case, we assign the page to the pipebuffer (fuse_ref_page()), so if the migration/compaction happens on the page before the server has read this page from the pipebuffer, it'll be incorrect data or maybe crash the kernel. > > > > > >>>> > >>> > >>> I'm still not seeing a good way around this. > >>> > >>> What about this then? We add a new fuse sysctl called something like > >>> "/proc/sys/fs/fuse/writeback_optimization_timeout" where if the sys > >>> admin sets this, then it opts into optimizing writeback to be as fast > >>> as possible (eg skipping the page copies) and if the server doesn't > >>> fulfill the writeback by the set timeout value, then the connection is > >>> aborted. > >>> > >>> Alternatively, we could also repurpose > >>> /proc/sys/fs/fuse/max_request_timeout from the request timeout > >>> patchset [1] but I like the additional flexibility and explicitness > >>> having the "writeback_optimization_timeout" sysctl gives. > >>> > >>> Any thoughts on this? > >> > >> > >> I'm a bit worried that we might lock up the system until time out is > >> reached - not ideal. Especially as timeouts are in minutes now. But > >> even a slightly stuttering video system not be great. I think we > >> should give users/admin the choice then, if they prefer slow page > >> copies or fast, but possibly shortly unresponsive system. > >> > > I was thinking the /proc/sys/fs/fuse/writeback_optimization_timeout > > would be in seconds, where the sys admin would probably set something > > more reasonable like 5 seconds or so. > > If this syctl value is set, then servers who want writebacks to be > > fast can opt into it at mount time (and by doing so agree that they > > will service writeback requests by the timeout or their connection > > will be aborted). > > > I think your current patch set has it in minutes? (Should be easy > enough to change that.) Though I'm more worried about the impact > of _frequent_ timeout scanning through the different fuse lists > on performance, than about missing compaction for folios that are > currently in write-back. > Ah, for this the " /proc/sys/fs/fuse/writeback_optimization_timeout" would be a separate thing from the "/proc/sys/fs/fuse/max_request_timeout". The "/proc/sys/fs/fuse/writeback_optimization_timeout" would only apply for writeback requests. I was thinking implementation-wise, for writebacks we could just have a timer associated with each request (instead of having to grab locks with the fuse lists), since they won't be super common. Thanks, Joanne > > Thanks, > Bernd
