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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Wed, 24 Apr 2019 12:44:50 +0200
Message-ID: <CAJfpegv0NvcsD9gh3aN+ngzsLqbNWUc73ZPN1qHox_14eN9Pqg@mail.gmail.com>
Subject: Re: [RESEND4, PATCH 1/2] fuse: retrieve: cap requested size to
 negotiated max_write
To:     Kirill Smelkov <kirr@nexedi.com>
Cc:     Miklos Szeredi <mszeredi@redhat.com>,
        Han-Wen Nienhuys <hanwen@google.com>,
        Jakob Unterwurzacher <jakobunt@gmail.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        fuse-devel <fuse-devel@lists.sourceforge.net>,
        stable <stable@vger.kernel.org>
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On Wed, Mar 27, 2019 at 11:15 AM Kirill Smelkov <kirr@nexedi.com> wrote:
>
> FUSE filesystem server and kernel client negotiate during initialization
> phase, what should be the maximum write size the client will ever issue.
> Correspondingly the filesystem server then queues sys_read calls to read
> requests with buffer capacity large enough to carry request header
> + that max_write bytes. A filesystem server is free to set its max_write
> in anywhere in the range between [1=C2=B7page, fc->max_pages=C2=B7page]. =
In
> particular go-fuse[2] sets max_write by default as 64K, wheres default
> fc->max_pages corresponds to 128K. Libfuse also allows users to
> configure max_write, but by default presets it to possible maximum.
>
> If max_write is < fc->max_pages=C2=B7page, and in NOTIFY_RETRIEVE handler=
 we
> allow to retrieve more than max_write bytes, corresponding prepared
> NOTIFY_REPLY will be thrown away by fuse_dev_do_read, because the
> filesystem server, in full correspondence with server/client contract,
> will be only queuing sys_read with ~max_write buffer capacity, and
> fuse_dev_do_read throws away requests that cannot fit into server
> request buffer. In turn the filesystem server could get stuck waiting
> indefinitely for NOTIFY_REPLY since NOTIFY_RETRIEVE handler returned OK
> which is understood by clients as that NOTIFY_REPLY was queued and will
> be sent back.
>
> -> Cap requested size to negotiate max_write to avoid the problem.
> This aligns with the way NOTIFY_RETRIEVE handler works, which already
> unconditionally caps requested retrieve size to fuse_conn->max_pages.
> This way it should not hurt NOTIFY_RETRIEVE semantic if we return less
> data than was originally requested.
>
> Please see [1] for context where the problem of stuck filesystem was hit
> for real, how the situation was traced and for more involving patch that
> did not make it into the tree.
>
> [1] https://marc.info/?l=3Dlinux-fsdevel&m=3D155057023600853&w=3D2
> [2] https://github.com/hanwen/go-fuse
>
> Signed-off-by: Kirill Smelkov <kirr@nexedi.com>
> Cc: Han-Wen Nienhuys <hanwen@google.com>
> Cc: Jakob Unterwurzacher <jakobunt@gmail.com>
> Cc: <stable@vger.kernel.org> # v2.6.36+
> ---
>  fs/fuse/dev.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 8a63e52785e9..38e94bc43053 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -1749,7 +1749,7 @@ static int fuse_retrieve(struct fuse_conn *fc, stru=
ct inode *inode,
>         offset =3D outarg->offset & ~PAGE_MASK;
>         file_size =3D i_size_read(inode);
>
> -       num =3D outarg->size;
> +       num =3D min(outarg->size, fc->max_write);

This is wrong: the max_size limited num is overwritten if constrained
by file size.

Also the patch is whitespace damaged.

Thanks,
Miklos