Re: [PATCH v1 00/16] NFS/RDMA patches proposed for 4.1

[Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Wed, May 06, 2015 at 09:09:36AM +0200, Bart Van Assche wrote:
> On 05/05/15 23:06, Christoph Hellwig wrote:
> >The contiguous requirements isn't something we can alway guarantee.
> >While a lot of I/O will have that form the form where there are holes
> >can happen, although it's not common.
> 
> Indeed. That is why there is code in the SRP initiator that uses multiple
> FRWR registrations when a discontiguous SG-list is passed to that driver by
> the SCSI mid-layer. Some time ago I had posted a test program that makes the
> SCSI mid-layer submit a discontiguous I/O request to a SCSI LLD (see also
> https://www.mail-archive.com/linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org/msg21224.html).

Note that block layer now has the (fairly misnamed) QUEUE_FLAG_SG_GAPS
flag, which ensures drivers don't see S/G lists like this one.  I
haven't ever seen normal TYPE_FS producers submit bios like that, so in
practice the effect is to reject vectored SG_IO uses like your test
program.
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