Re: [PATCH] xen/blkfront: Only check REQ_FUA for writes

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Apr 26, 2023 at 05:40:05PM +0100, Ross Lagerwall wrote:
> The existing code silently converts read operations with the
> REQ_FUA bit set into write-barrier operations. This results in data
> loss as the backend scribbles zeroes over the data instead of returning
> it.
> 
> While the REQ_FUA bit doesn't make sense on a read operation, at least
> one well-known out-of-tree kernel module does set it and since it
> results in data loss, let's be safe here and only look at REQ_FUA for
> writes.

Do we know what's the intention of the out-of-tree kernel module with
it's usage of FUA for reads?

Should this maybe be translated to a pair of flush cache and read
requests?

> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> ---
>  drivers/block/xen-blkfront.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 23ed258b57f0..c1890c8a9f6e 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -780,7 +780,8 @@ static int blkif_queue_rw_req(struct request *req, struct 
> blkfront_ring_info *ri
>               ring_req->u.rw.handle = info->handle;
>               ring_req->operation = rq_data_dir(req) ?
>                       BLKIF_OP_WRITE : BLKIF_OP_READ;
> -             if (req_op(req) == REQ_OP_FLUSH || req->cmd_flags & REQ_FUA) {
> +             if (req_op(req) == REQ_OP_FLUSH ||
> +                 (req_op(req) == REQ_OP_WRITE && (req->cmd_flags & 
> REQ_FUA))) {

Should we print some kind of warning maybe once that we have received
a READ request with the FUA flag set, and the FUA flag will have no
effect?

Thanks, Roger.