linux-kernel - Re: [PATCH 5.15] fuse: Fix race condition in writethrough path A race

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Message-ID: <CAJfpegs85DzZjzyCNQ+Lh8R2cLDBG=GcMbEfr5PGSS531hxAeA@mail.gmail.com>
Date: Mon, 13 Oct 2025 15:39:48 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: lu gu <giveme.gulu@...il.com>
Cc: Joanne Koong <joannelkoong@...il.com>, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Brian Foster <bfoster@...hat.com>, 
	Bernd Schubert <bernd@...ernd.com>
Subject: Re: [PATCH 5.15] fuse: Fix race condition in writethrough path A race

On Fri, 10 Oct 2025 at 10:46, Miklos Szeredi <miklos@...redi.hu> wrote:

> My idea is to introduce FUSE_I_MTIME_UNSTABLE (which would work
> similarly to FUSE_I_SIZE_UNSTABLE) and when fetching old_mtime, verify
> that it hasn't been invalidated.  If old_mtime is invalid or if
> FUSE_I_MTIME_UNSTABLE signals that a write is in progress, the page
> cache is not invalidated.

[Adding Brian Foster, the author of FUSE_AUTO_INVAL_DATA patches.
Link to complete thread:
https://lore.kernel.org/all/20251009110623.3115511-1-giveme.gulu@gmail.com/#r]

In summary: auto_inval_data invalidates data cache even if the
modification was done in a cache consistent manner (i.e. write
through). This is not generally a consistency problem, because the
backing file and the cache should be in sync.  The exception is when
the writeback to the backing file hasn't yet finished and a getattr()
call triggers invalidation (mtime change could be from a previous
write), and the not yet written data is invalidated and replaced with
stale data.

The proposed fix was to exclude concurrent reads and writes to the same region.

But the real issue here is that mtime changes triggered by this client
should not cause data to be invalidated.  It's not only racy, but it's
fundamentally wrong.  Unfortunately this is hard to do this correctly.
Best I can come up with is that any request that expects mtime to be
modified returns the mtime after the request has completed.

This would be much easier to implement in the fuse server: perform the
"file changed remotely" check when serving a FUSE_GETATTR request and
return a flag indicating whether the data needs to be invalidated or
not.

Thoughts?

Thanks,
Miklos