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Date: Wed, 26 Oct 2022 11:01:50 +0200 From: Miklos Szeredi <miklos@...redi.hu> To: Tycho Andersen <tycho@...ho.pizza> Cc: Eric Biederman <ebiederm@...ssion.com>, "Serge E. Hallyn" <serge@...lyn.com>, linux-kernel@...r.kernel.org, fuse-devel@...ts.sourceforge.net Subject: Re: [PATCH v2] fuse: In fuse_flush only wait if someone wants the return code On Fri, 30 Sept 2022 at 18:10, Tycho Andersen <tycho@...ho.pizza> wrote: > > On Fri, Sep 30, 2022 at 04:41:37PM +0200, Miklos Szeredi wrote: > > On Fri, 30 Sept 2022 at 16:01, Tycho Andersen <tycho@...ho.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@...ho.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(). > > Agreed. > > > 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. > > How can we wait in fuse_set_nowrite()? Or are you suggesting we just > do a fuse_flush_writepages() in the async part and hope for the best? I was thinking along the lines of calling schedule_work() in the exiting case to do the flush. Thanks, Miklos
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