linux-kernel - Re: fuse: fuse semantics wrt stalled requests

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Message-ID: <45da8248-d694-4220-a120-3d85a463c0cf@fastmail.fm>
Date: Thu, 28 Nov 2024 11:29:40 +0100
From: Bernd Schubert <bernd.schubert@...tmail.fm>
To: Sergey Senozhatsky <senozhatsky@...omium.org>,
 Miklos Szeredi <miklos@...redi.hu>
Cc: Tomasz Figa <tfiga@...omium.org>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: fuse: fuse semantics wrt stalled requests



On 11/28/24 04:54, Sergey Senozhatsky wrote:
> Hello Miklos,
> 
> A question: does fuse define any semantics for stalled requests handling?
> 
> We are currently looking at a number of hung_task watchdog crashes with
> tasks waiting forever in d_wait_lookup() for dentries to lose PAR_LOOKUP
> state, and we suspect that those dentries are from fuse mount point
> (we also sometimes see hung_tasks in fuse_lookup()->fuse_simple_request()).
> Supposedly (a theory) some tasks are in request_wait_answer() under
> PAR_LOOKUP, and the rest of tasks are waiting for them to finish and clear
> PAR_LOOKUP bit.
> 
> request_wait_answer() waits indefinitely, however, the interesting
> thing is that it uses wait_event_interruptible() (when we wait for
> !fc->no_interrupt request to be processed).  What is the idea behind
> interruptible wait?  Is this, may be, for stall requests handling?
> Does fuse expect user-space to watchdog or monitor its processes/threads
> that issue syscalls on fuse mount points and, e.g., SIGKILL stalled ones?
> 
> To make things even more complex, in our particular case fuse mount
> point mounts a remote google driver, so it become a network fs in
> some sense, which adds a whole new dimension of possibilities for
> stalled/failed requests.  How those are expected to be handled?  Should
> fuse still wait indefinitely or would it make sense to add a timeout
> to request_wait_answer() and FR_INTERRUPTED timeout-ed requests?
> 

Please see here

https://lore.kernel.org/all/20241114191332.669127-1-joannelkoong@gmail.com/



Thanks,
Bernd