Ceph » Linux kernel client

rbd requires stable pages if ceph message data CRCs are enabled.  To
that end we set BDI_CAP_STABLE_WRITES in rbd_init_disk(), but since
commit 25520d55cdb6 ("block: Inline blk_integrity in struct gendisk",
went into 4.4) this bit is almost immediately cleared:

add_disk
  register_disk
    blkdev_get
      rescan_partitions
        blk_integrity_revalidate

void blk_integrity_revalidate(struct gendisk *disk)
{
        struct blk_integrity *bi = &disk->queue->integrity;

        if (!(disk->flags & GENHD_FL_UP))
                return;

        if (bi->profile)
                disk->queue->backing_dev_info.capabilities |=
                        BDI_CAP_STABLE_WRITES;
        else
                disk->queue->backing_dev_info.capabilities &=
                        ~BDI_CAP_STABLE_WRITES;
}

ceph messenger is responsible for generating/verifying CRCs, so we
don't call blk_integrity_register() -- bi->profile is NULL.

More at http://www.spinics.net/lists/ceph-devel/msg35413.html.