Re: [Xen-devel] [PATCH] xen/blkfront: improve protection against issuing unsupported REQ_FUA

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/03/2014 07:22 AM, Laszlo Ersek wrote:
> On 10/27/14 14:44, Vitaly Kuznetsov wrote:
> > Guard against issuing unsupported REQ_FUA and REQ_FLUSH was introduced
> > in d11e61583 and was factored out into blkif_request_flush_valid() in
> > 0f1ca65ee. However:
> > 1) This check in incomplete. In case we negotiated to feature_flush = REQ_FLUSH
> >     and flush_op = BLKIF_OP_FLUSH_DISKCACHE (so FUA is unsupported) FUA request
> >     will still pass the check.
> > 2) blkif_request_flush_valid() is misnamed. It is bool but returns true when
> >     the request is invalid.
> > 3) When blkif_request_flush_valid() fails -EIO is being returned. It seems that
> >     -EOPNOTSUPP is more appropriate here.
> > Fix all of the above issues.
> >
> > This patch is based on the original patch by Laszlo Ersek and a comment by
> > Jeff Moyer.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > ---
> >   drivers/block/xen-blkfront.c | 14 ++++++++------
> >   1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> > index 5ac312f..2e6c103 100644
> > --- a/drivers/block/xen-blkfront.c
> > +++ b/drivers/block/xen-blkfront.c
> > @@ -582,12 +582,14 @@ static inline void flush_requests(struct blkfront_info 
> > *info)
> >                 notify_remote_via_irq(info->irq);
> >   }
> >   
> > -static inline bool blkif_request_flush_valid(struct request *req,
> > -                                            struct blkfront_info *info)
> > +static inline bool blkif_request_flush_invalid(struct request *req,
> > +                                              struct blkfront_info *info)
> >   {
> >         return ((req->cmd_type != REQ_TYPE_FS) ||
> > -               ((req->cmd_flags & (REQ_FLUSH | REQ_FUA)) &&
> > -               !info->flush_op));
> > +               ((req->cmd_flags & REQ_FLUSH) &&
> > +                !(info->feature_flush & REQ_FLUSH)) ||
> > +               ((req->cmd_flags & REQ_FUA) &&
> > +                !(info->feature_flush & REQ_FUA)));
Somewhat unrelated to the patch, but I am wondering whether we actually 
need flush_op field at all as it seems that it is unambiguously defined 
by REQ_FLUSH/REQ_FUA.
-boris

> >   }
> >   
> >   /*
> > @@ -612,8 +614,8 @@ static void do_blkif_request(struct request_queue *rq)
> >   
> >   		blk_start_request(req);
> >   
> > -		if (blkif_request_flush_valid(req, info)) {
> > -                       __blk_end_request_all(req, -EIO);
> > +               if (blkif_request_flush_invalid(req, info)) {
> > +                       __blk_end_request_all(req, -EOPNOTSUPP);
> >                         continue;
> >                 }
> >   
> Not sure if there has been some feedback yet (I can't see anything
> threaded with this message in my inbox).
>
> FWIW I consulted "Documentation/block/writeback_cache_control.txt" for
> this review. Apparently, REQ_FLUSH forces out "previously completed
> write requests", whereas REQ_FUA delays the IO completion signal for
> *this* request until "the data has been committed to non-volatile
> storage". So, indeed, support for REQ_FLUSH only does not guarantee that
> REQ_FUA can be served.
>
> Reviewed-by: Laszlo Ersek <lersek@xxxxxxxxxx>
>
> Thanks
> Laszlo

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