Re: [PATCH] block: bios with an offset are always gappy

[Ming Lei]

On Thu, Apr 13, 2017 at 01:53:28PM +0200, Johannes Thumshirn wrote:
> On Thu, Apr 13, 2017 at 06:02:21PM +0800, Ming Lei wrote:
> > On Thu, Apr 13, 2017 at 10:06:29AM +0200, Johannes Thumshirn wrote:
> > > Doing a mkfs.btrfs on a (qemu emulated) PCIe NVMe causes a kernel panic
> > > in nvme_setup_prps() because the dma_len will drop below zero but the
> > > length not.
> > 
> > Looks I can't reproduce the issue in QEMU(32G nvme, either partitioned
> > or not, just use 'mkfs.btrfs /dev/nvme0n1p1'), could you share the exact
> > mkfs command line and size of your emulated NVMe?
> 
> the exact cmdline is mkfs.btrfs -f /dev/nvme0n1p1 (-f because there was a
> existing btrfs on the image). The image is 17179869184 (a.k.a 16G) bytes.
> 
> [...]
> 
> > Could you try the following patch to see if it fixes your issue?
> 
> It's back to the old, erratic behaviour, see log below.

Johannes, could you test the following patch?

Thanks
Ming

---

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index b75d6fe5a1b9..cc68dfaef528 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1672,12 +1672,27 @@ static inline bool bios_segs_mergeable(struct 
request_queue *q,
        return true;
 }
 
-static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
-                        struct bio *next)
+static inline bool bio_will_gap(struct request_queue *q,
+                               struct request *prev_rq,
+                               struct bio *prev,
+                               struct bio *next)
 {
        if (bio_has_data(prev) && queue_virt_boundary(q)) {
                struct bio_vec pb, nb;
 
+               /*
+                * don't merge if the 1st bio starts with non-zero
+                * offset, otherwise it is quite difficult to respect
+                * sg gap limit. We work hard to merge huge of small
+                * bios in case of mkfs.
+                */
+               if (prev_rq)
+                       bio_get_first_bvec(prev_rq->bio, &pb);
+               else
+                       bio_get_first_bvec(prev, &pb);
+               if (pb.bv_offset)
+                       return true;
+
                bio_get_last_bvec(prev, &pb);
                bio_get_first_bvec(next, &nb);
 
@@ -1690,12 +1705,12 @@ static inline bool bio_will_gap(struct request_queue 
*q, struct bio *prev,
 
 static inline bool req_gap_back_merge(struct request *req, struct bio *bio)
 {
-       return bio_will_gap(req->q, req->biotail, bio);
+       return bio_will_gap(req->q, req, req->biotail, bio);
 }
 
 static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
 {
-       return bio_will_gap(req->q, bio, req->bio);
+       return bio_will_gap(req->q, NULL, bio, req->bio);
 }
 
 static inline void blk_dump_rq(const struct request *req, const char *msg)