On 10/25/24 12:54 AM, Joanne Koong wrote: > On Mon, Oct 21, 2024 at 2:05 PM Joanne Koong <joannelkoong@xxxxxxxxx> wrote: >> >> On Mon, Oct 21, 2024 at 3:15 AM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: >>> >>> On Fri, 18 Oct 2024 at 07:31, Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: >>> >>>> I feel like this is too much restrictive and I am still not sure why >>>> blocking on fuse folios served by non-privileges fuse server is worse >>>> than blocking on folios served from the network. >>> >>> Might be. But historically fuse had this behavior and I'd be very >>> reluctant to change that unconditionally. >>> >>> With a systemwide maximal timeout for fuse requests it might make >>> sense to allow sync(2), etc. to wait for fuse writeback. >>> >>> Without a timeout allowing fuse servers to block sync(2) indefinitely >>> seems rather risky. >> >> Could we skip waiting on writeback in sync(2) if it's a fuse folio? >> That seems in line with the sync(2) documentation Jingbo referenced >> earlier where it states "The writing, although scheduled, is not >> necessarily complete upon return from sync()." >> https://pubs.opengroup.org/onlinepubs/9699919799/functions/sync.html >> > > So I think the answer to this is "no" for Linux. What the Linux man > page for sync(2) says: > > "According to the standard specification (e.g., POSIX.1-2001), sync() > schedules the writes, but may return before the actual writing is > done. However Linux waits for I/O completions, and thus sync() or > syncfs() provide the same guarantees as fsync() called on every file > in the system or filesystem respectively." [1] Actually as for FUSE, IIUC the writeback is not guaranteed to be completed when sync(2) returns since the temp page mechanism. When sync(2) returns, PG_writeback is indeed cleared for all original pages (in the address_space), while the real writeback work (initiated from temp page) may be still in progress. I think this is also what Miklos means in: https://lore.kernel.org/all/CAJfpegsJKD4YT5R5qfXXE=hyqKvhpTRbD4m1wsYNbGB6k4rC2A@xxxxxxxxxxxxxx/ Though we need special handling for AS_NO_WRITEBACK_RECLAIM marked pages in sync(2) codepath similar to what we have done for the direct reclaim in patch 1. > > Regardless of the compaction / page migration issue then, this > blocking sync(2) is a dealbreaker. I really should have figureg out the compaction / page migration mechanism and the potential impact to FUSE when we dropping the temp page. Just too busy to take some time on this though..... -- Thanks, Jingbo
