linux-kernel - Re: [PATCH 2/3] virtiofs: split requests that exceed virtqueue size

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Date:   Mon, 22 Mar 2021 15:47:37 +0000
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Connor Kuehl <ckuehl@...hat.com>
Cc:     virtio-fs@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, vgoyal@...hat.com,
        miklos@...redi.hu, jasowang@...hat.com, mst@...hat.com
Subject: Re: [PATCH 2/3] virtiofs: split requests that exceed virtqueue size

On Thu, Mar 18, 2021 at 08:52:22AM -0500, Connor Kuehl wrote:
> If an incoming FUSE request can't fit on the virtqueue, the request is
> placed onto a workqueue so a worker can try to resubmit it later where
> there will (hopefully) be space for it next time.
> 
> This is fine for requests that aren't larger than a virtqueue's maximum
> capacity. However, if a request's size exceeds the maximum capacity of
> the virtqueue (even if the virtqueue is empty), it will be doomed to a
> life of being placed on the workqueue, removed, discovered it won't fit,
> and placed on the workqueue yet again.
> 
> Furthermore, from section 2.6.5.3.1 (Driver Requirements: Indirect
> Descriptors) of the virtio spec:
> 
>   "A driver MUST NOT create a descriptor chain longer than the Queue
>   Size of the device."
> 
> To fix this, limit the number of pages FUSE will use for an overall
> request. This way, each request can realistically fit on the virtqueue
> when it is decomposed into a scattergather list and avoid violating
> section 2.6.5.3.1 of the virtio spec.
> 
> Signed-off-by: Connor Kuehl <ckuehl@...hat.com>
> ---
>  fs/fuse/fuse_i.h    |  5 +++++
>  fs/fuse/inode.c     |  7 +++++++
>  fs/fuse/virtio_fs.c | 14 ++++++++++++++
>  3 files changed, 26 insertions(+)

Nice that FUSE already has max_pages :-).

Reviewed-by: Stefan Hajnoczi <stefanha@...hat.com>

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