[dm-devel] [patch] dm: propagate QUEUE_FLAG_NO_SG_MERGE

[Greg KH]

On Fri, Aug 08, 2014 at 11:03:41AM -0400, Jeff Moyer wrote:
> Hi,
> 
> Commit 05f1dd5 introduced a new queue flag: QUEUE_FLAG_NO_SG_MERGE.
> This gets set by default in blk_mq_init_queue for mq-enabled devices.
> The effect of the flag is to bypass the SG segment merging.  Instead,
> the bio->bi_vcnt is used as the number of hardware segments.
> 
> With a device mapper target on top of a device with
> QUEUE_FLAG_NO_SG_MERGE set, we can end up sending down more segments
> than a driver is prepared to handle.  I ran into this when backporting
> the virtio_blk mq support.  It triggerred this BUG_ON, in
> virtio_queue_rq:
> 
>         BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
> 
> The queue's max is set here:
>         blk_queue_max_segments(q, vblk->sg_elems-2);
> 
> Basically, what happens is that a bio is built up for the dm device
> (which does not have the QUEUE_FLAG_NO_SG_MERGE flag set) using
> bio_add_page.  That path will call into __blk_recalc_rq_segments, so
> what you end up with is bi_phys_segments being much smaller than bi_vcnt
> (and bi_vcnt grows beyond the maximum sg elements).  Then, when the bio
> is submitted, it gets cloned.  When the cloned bio is submitted, it will
> end up in blk_recount_segments, here:
> 
>         if (test_bit(QUEUE_FLAG_NO_SG_MERGE, &q->queue_flags))
>                 bio->bi_phys_segments = bio->bi_vcnt;
> 
> and now we've set bio->bi_phys_segments to a number that is beyond what
> was registered as queue_max_segments by the driver.
> 
> The right way to fix this is to propagate the queue flag up the stack.
> Attached is a patch that does this, tested and confirmed to fix the
> problem in my environment.
> 
> The rules for propagating the flag are simple:
> - if the flag is set for any underlying device, it must be set for the
>   upper device
> - consequently, if the flag is not set for any underlying device, it
>   should not be set for the upper device.
> 
> stable notes: this patch should be applied to 3.16.
> 
> Signed-off-by: Jeff Moyer <jmoyer at redhat.com>
> 

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read Documentation/stable_kernel_rules.txt
for how to do this properly.

</formletter>