linux-kernel - Re: [PATCH RESEND V11 0/7] fuse: Add support for passthrough read/write

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Date:   Tue, 19 Jan 2021 12:34:23 +0000
From:   Alessio Balsini <balsini@...roid.com>
To:     Rokudo Yan <wu-yan@....com>
Cc:     balsini@...roid.com, akailash@...gle.com, amir73il@...il.com,
        axboe@...nel.dk, bergwolf@...il.com, duostefano93@...il.com,
        dvander@...gle.com, fuse-devel@...ts.sourceforge.net,
        gscrivan@...hat.com, jannh@...gle.com, kernel-team@...roid.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        maco@...roid.com, miklos@...redi.hu, palmer@...belt.com,
        paullawrence@...gle.com, trapexit@...wn.link, zezeozue@...gle.com
Subject: Re: [PATCH RESEND V11 0/7] fuse: Add support for passthrough
 read/write

On Tue, Jan 19, 2021 at 07:06:54PM +0800, Rokudo Yan wrote:
> on Mon, Jan 18, 2021 at 5:27 PM Alessio Balsini <balsini@...roid.com> wrote:
> >
> > This is the 11th version of the series, rebased on top of v5.11-rc4.
> > Please find the changelog at the bottom of this cover letter.
> > 
> > Add support for file system passthrough read/write of files when enabled
> > in userspace through the option FUSE_PASSTHROUGH.
> [...]
> 
> 
> Hi Allesio,
> 
> Could you please add support for passthrough mmap too ?
> If the fuse file opened with passthrough actived, and then map (shared) to (another) process
> address space using mmap interface. As access the file with mmap will pass the vfs cache of fuse,
> but access the file with read/write will bypass the vfs cache of fuse, this may cause inconsistency.
> eg. the reader read the fuse file with mmap() and the writer modify the file with write(), the reader
> may not see the modification immediately since the writer bypass the vfs cache of fuse.
> Actually we have already meet an issue caused by the inconsistency after applying fuse passthrough
> scheme to our product.
> 
> Thanks,
> yanwu.

Hi yanwu,

Thank you for your interest in this change.

FUSE passthrough for mmap is an extension that is already in my TODO
list, together with passthrough for directories.
For now I would prefer to keep this series minimal to make the review
process leaner and simpler.
I will start working on extending this series with new features and
addressing more corner cases as soon as these changes get merged, what
do you think?

Thanks,
Alessio