Messages in this thread | | | From | Miklos Szeredi <> | Date | Fri, 30 Sep 2022 16:41:37 +0200 | Subject | Re: [PATCH v2] fuse: In fuse_flush only wait if someone wants the return code |
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On Fri, 30 Sept 2022 at 16:01, Tycho Andersen <tycho@tycho.pizza> wrote: > > On Fri, Sep 30, 2022 at 03:35:16PM +0200, Miklos Szeredi wrote: > > On Thu, 29 Sept 2022 at 18:40, Tycho Andersen <tycho@tycho.pizza> wrote: > > > > > > If a fuse filesystem is mounted inside a container, there is a problem > > > during pid namespace destruction. The scenario is: > > > > > > 1. task (a thread in the fuse server, with a fuse file open) starts > > > exiting, does exit_signals(), goes into fuse_flush() -> wait > > > > Can't the same happen through > > > > fuse_flush -> fuse_sync_writes -> fuse_set_nowrite -> wait > > > > ? > > Looks like yes, though I haven't seen this in the wild, I guess > because there aren't multiple writers most of the time the user code > that causes this. > > I'm not exactly sure how to fix this. Reading through 3be5a52b30aa > ("fuse: support writable mmap"), we don't want to allow multiple > writes since that may do allocations, which could cause deadlocks. But > in this case we have no reliable way to wait (besides a busy loop, I > suppose). > > Maybe just a check for PF_EXITING and a pr_warn() with "echo 1 > > /sys/fs/fuse/connections/$N/abort" or something?
AFAICS it should be perfectly normal (and trivial to trigger) for an exiting process to have its dirty pages flushed through fuse_flush().
We could do that asynchronously as well, generally there are no promises about dirty pages being synced as part of the process exiting . But ordering between dirty page flushing and sending the FUSE_FLUSH request should be kept. Which needs more complexity, unfortunately.
Thanks, Miklos
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