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[Xen-devel] [patch] barrier support for blk{front,back}

To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] [patch] barrier support for blk{front,back}
From: Gerd Hoffmann <kraxel@xxxxxxx>
Date: Fri, 08 Sep 2006 10:34:30 -0700
Delivery-date: Fri, 08 Sep 2006 10:24:52 -0700
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  Hi,

This patch adds support for barriers to blk{back,front} drivers.

protocol changes:
 * There is a new operation (BLKIF_OP_WRITE_BARRIER)
   to pass on barrier requests.
 * There is a new state (BLKIF_RSP_EOPNOTSUPP) to indicate
   unsupported operations (barrier writes may fail depending
   on the underlying block device).
 * A new xenstore node named "feature-barrier" indicates the
   backend is able to handle barrier writes.  The value can
   be 1 (all is fine) or 0 (underlying block device doesn't
   support barriers).

blkback changes:  Add "feature-barrier" node to indicate barrier
support, pass incoming barrier requests to the block layer using
submit_bio(WRITE_BARRIER, bio).  Some error handling fixes to
properly pass through barrier write failures, so the frontend
can turn off barriers then.

blkfront changes:  Check if the backend sets "feature-barrier", if
present switch to QUEUE_ORDERED_DRAIN mode.  Send off barrier
requests to the backend using the new BLKIF_OP_WRITE_BARRIER
operation.  Also some error handling for the EOPNOTSUPP case.

cheers,

  Gerd

-- 
Gerd Hoffmann <kraxel@xxxxxxx>
Add support for barriers to blk{back,front} drivers.

protocol changes:
 * There is a new operation (BLKIF_OP_WRITE_BARRIER)
   to pass on barrier requests.
 * There is a new state (BLKIF_RSP_EOPNOTSUPP) to indicate
   unsupported operations (barrier writes may fail depending
   on the underlying block device).
 * A new xenstore node named "feature-barrier" indicates the
   backend is able to handle barrier writes.  The value can
   be 1 (all is fine) or 0 (underlying block device doesn't
   support barriers).

blkback changes:  Add "feature-barrier" node to indicate barrier
support, pass incoming barrier requests to the block layer using
submit_bio(WRITE_BARRIER, bio).  Some error handling fixes to
properly pass through barrier write failures, so the frontend
can turn off barriers then.

blkfront changes:  Check if the backend sets "feature-barrier", if
present switch to QUEUE_ORDERED_DRAIN mode.  Send off barrier
requests to the backend using the new BLKIF_OP_WRITE_BARRIER
operation.  Also some error handling for the EOPNOTSUPP case.

Signed-off-by: Gerd Hoffmann <kraxel@xxxxxxx>
---
 linux-2.6-xen-sparse/drivers/xen/blkback/blkback.c   |   41 ++++++++++++++-----
 linux-2.6-xen-sparse/drivers/xen/blkback/common.h    |    3 +
 linux-2.6-xen-sparse/drivers/xen/blkback/vbd.c       |    2 
 linux-2.6-xen-sparse/drivers/xen/blkback/xenbus.c    |   31 ++++++++++++++
 linux-2.6-xen-sparse/drivers/xen/blkfront/blkfront.c |   30 +++++++++++--
 linux-2.6-xen-sparse/drivers/xen/blkfront/block.h    |    2 
 linux-2.6-xen-sparse/drivers/xen/blkfront/vbd.c      |   20 ++++++++-
 xen/include/public/io/blkif.h                        |   10 ++--
 8 files changed, 117 insertions(+), 22 deletions(-)

Index: 
build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/blkback.c
===================================================================
--- 
build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkback/blkback.c
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/blkback.c
@@ -191,9 +191,9 @@ static void fast_flush_area(pending_req_
 
 static void print_stats(blkif_t *blkif)
 {
-       printk(KERN_DEBUG "%s: oo %3d  |  rd %4d  |  wr %4d\n",
+       printk(KERN_DEBUG "%s: oo %3d  |  rd %4d  |  wr %4d  |  br %4d\n",
               current->comm, blkif->st_oo_req,
-              blkif->st_rd_req, blkif->st_wr_req);
+              blkif->st_rd_req, blkif->st_wr_req, blkif->st_br_req);
        blkif->st_print = jiffies + msecs_to_jiffies(10 * 1000);
        blkif->st_rd_req = 0;
        blkif->st_wr_req = 0;
@@ -243,12 +243,17 @@ int blkif_schedule(void *arg)
  * COMPLETION CALLBACK -- Called as bh->b_end_io()
  */
 
-static void __end_block_io_op(pending_req_t *pending_req, int uptodate)
+static void __end_block_io_op(pending_req_t *pending_req, int error)
 {
        /* An error fails the entire request. */
-       if (!uptodate) {
-               DPRINTK("Buffer not up-to-date at end of operation\n");
+       if (error) {
+               DPRINTK("Buffer not up-to-date at end of operation, 
error=%d\n", error);
                pending_req->status = BLKIF_RSP_ERROR;
+               if (pending_req->operation == BLKIF_OP_WRITE_BARRIER  &&  error 
== -EOPNOTSUPP) {
+                       pending_req->status = BLKIF_RSP_EOPNOTSUPP;
+                       blkback_barrier(pending_req->blkif->be, 0);
+                       printk("blkback: write barrier op failed, not 
supported\n");
+               }
        }
 
        if (atomic_dec_and_test(&pending_req->pendcnt)) {
@@ -264,7 +269,7 @@ static int end_block_io_op(struct bio *b
 {
        if (bio->bi_size != 0)
                return 1;
-       __end_block_io_op(bio->bi_private, !error);
+       __end_block_io_op(bio->bi_private, error);
        bio_put(bio);
        return error;
 }
@@ -321,6 +326,9 @@ static int do_block_io_op(blkif_t *blkif
                        blkif->st_rd_req++;
                        dispatch_rw_block_io(blkif, req, pending_req);
                        break;
+               case BLKIF_OP_WRITE_BARRIER:
+                       blkif->st_br_req++;
+                       /* fall through */
                case BLKIF_OP_WRITE:
                        blkif->st_wr_req++;
                        dispatch_rw_block_io(blkif, req, pending_req);
@@ -342,7 +350,6 @@ static void dispatch_rw_block_io(blkif_t
                                 pending_req_t *pending_req)
 {
        extern void ll_rw_block(int rw, int nr, struct buffer_head * bhs[]);
-       int operation = (req->operation == BLKIF_OP_WRITE) ? WRITE : READ;
        struct gnttab_map_grant_ref map[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        struct phys_req preq;
        struct { 
@@ -351,6 +358,22 @@ static void dispatch_rw_block_io(blkif_t
        unsigned int nseg;
        struct bio *bio = NULL, *biolist[BLKIF_MAX_SEGMENTS_PER_REQUEST];
        int ret, i, nbio = 0;
+       int operation;
+
+       switch (req->operation) {
+       case BLKIF_OP_READ:
+               operation = READ;
+               break;
+       case BLKIF_OP_WRITE:
+               operation = WRITE;
+               break;
+       case BLKIF_OP_WRITE_BARRIER:
+               operation = WRITE_BARRIER;
+               break;
+       default:
+               operation = 0; /* make gcc happy */
+               BUG();
+       }
 
        /* Check that number of segments is sane. */
        nseg = req->nr_segments;
@@ -366,7 +389,7 @@ static void dispatch_rw_block_io(blkif_t
 
        pending_req->blkif     = blkif;
        pending_req->id        = req->id;
-       pending_req->operation = operation;
+       pending_req->operation = req->operation;
        pending_req->status    = BLKIF_RSP_OKAY;
        pending_req->nr_pages  = nseg;
 
@@ -382,7 +405,7 @@ static void dispatch_rw_block_io(blkif_t
                preq.nr_sects += seg[i].nsec;
 
                flags = GNTMAP_host_map;
-               if ( operation == WRITE )
+               if ( operation != READ )
                        flags |= GNTMAP_readonly;
                gnttab_set_map_op(&map[i], vaddr(pending_req, i), flags,
                                  req->seg[i].gref, blkif->domid);
Index: build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/common.h
===================================================================
--- 
build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkback/common.h
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/common.h
@@ -87,6 +87,7 @@ typedef struct blkif_st {
        int                 st_rd_req;
        int                 st_wr_req;
        int                 st_oo_req;
+       int                 st_br_req;
 
        wait_queue_head_t waiting_to_free;
 
@@ -131,4 +132,6 @@ void blkif_xenbus_init(void);
 irqreturn_t blkif_be_int(int irq, void *dev_id, struct pt_regs *regs);
 int blkif_schedule(void *arg);
 
+int blkback_barrier(struct backend_info *be, int state);
+
 #endif /* __BLKIF__BACKEND__COMMON_H__ */
Index: build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/xenbus.c
===================================================================
--- 
build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkback/xenbus.c
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/xenbus.c
@@ -91,11 +91,13 @@ static void update_blkif_status(blkif_t 
 VBD_SHOW(oo_req, "%d\n", be->blkif->st_oo_req);
 VBD_SHOW(rd_req, "%d\n", be->blkif->st_rd_req);
 VBD_SHOW(wr_req, "%d\n", be->blkif->st_wr_req);
+VBD_SHOW(br_req, "%d\n", be->blkif->st_br_req);
 
 static struct attribute *vbdstat_attrs[] = {
        &dev_attr_oo_req.attr,
        &dev_attr_rd_req.attr,
        &dev_attr_wr_req.attr,
+       &dev_attr_br_req.attr,
        NULL
 };
 
@@ -165,6 +167,31 @@ static int blkback_remove(struct xenbus_
        return 0;
 }
 
+int blkback_barrier(struct backend_info *be, int state)
+{
+       struct xenbus_device *dev = be->dev;
+       struct xenbus_transaction xbt;
+       int err;
+
+       do {
+               err = xenbus_transaction_start(&xbt);
+               if (err) {
+                       xenbus_dev_fatal(dev, err, "starting transaction");
+                       return -1;
+               }
+
+               err = xenbus_printf(xbt, dev->nodename, "feature-barrier",
+                                   "%d", state);
+               if (err) {
+                       xenbus_transaction_end(xbt, 1);
+                       xenbus_dev_fatal(dev, err, "writing feature-barrier");
+                       return -1;
+               }
+
+               err = xenbus_transaction_end(xbt, 0);
+       } while (err == -EAGAIN);
+       return 0;
+}
 
 /**
  * Entry point to this code when a new device is created.  Allocate the basic
@@ -201,6 +228,10 @@ static int blkback_probe(struct xenbus_d
        if (err)
                goto fail;
 
+       err = blkback_barrier(be, 1);
+       if (err)
+               goto fail;
+
        err = xenbus_switch_state(dev, XenbusStateInitWait);
        if (err)
                goto fail;
Index: 
build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkfront/blkfront.c
===================================================================
--- 
build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkfront/blkfront.c
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkfront/blkfront.c
@@ -316,6 +316,12 @@ static void connect(struct blkfront_info
                return;
        }
 
+       err = xenbus_gather(XBT_NIL, info->xbdev->otherend,
+                           "feature-barrier", "%lu", &info->feature_barrier,
+                           NULL);
+       if (err)
+               info->feature_barrier = 0;
+
        err = xlvbd_add(sectors, info->vdevice, binfo, sector_size, info);
        if (err) {
                xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
@@ -542,11 +548,14 @@ static int blkif_queue_request(struct re
        info->shadow[id].request = (unsigned long)req;
 
        ring_req->id = id;
-       ring_req->operation = rq_data_dir(req) ?
-               BLKIF_OP_WRITE : BLKIF_OP_READ;
        ring_req->sector_number = (blkif_sector_t)req->sector;
        ring_req->handle = info->handle;
 
+       ring_req->operation = rq_data_dir(req) ?
+               BLKIF_OP_WRITE : BLKIF_OP_READ;
+       if (blk_barrier_rq(req))
+               ring_req->operation = BLKIF_OP_WRITE_BARRIER;
+
        ring_req->nr_segments = 0;
        rq_for_each_bio (bio, req) {
                bio_for_each_segment (bvec, bio, idx) {
@@ -643,6 +652,7 @@ static irqreturn_t blkif_int(int irq, vo
        RING_IDX i, rp;
        unsigned long flags;
        struct blkfront_info *info = (struct blkfront_info *)dev_id;
+       int uptodate;
 
        spin_lock_irqsave(&blkif_io_lock, flags);
 
@@ -667,19 +677,27 @@ static irqreturn_t blkif_int(int irq, vo
 
                ADD_ID_TO_FREELIST(info, id);
 
+               uptodate = (bret->status == BLKIF_RSP_OKAY);
                switch (bret->operation) {
+               case BLKIF_OP_WRITE_BARRIER:
+                       if (unlikely(bret->status == BLKIF_RSP_EOPNOTSUPP)) {
+                               printk("blkfront: %s: write barrier op 
failed\n",
+                                      info->gd->disk_name);
+                               uptodate = -EOPNOTSUPP;
+                               info->feature_barrier = 0;
+                               xlvbd_barrier(info);
+                       }
+                       /* fall through */
                case BLKIF_OP_READ:
                case BLKIF_OP_WRITE:
                        if (unlikely(bret->status != BLKIF_RSP_OKAY))
                                DPRINTK("Bad return from blkdev data "
                                        "request: %x\n", bret->status);
 
-                       ret = end_that_request_first(
-                               req, (bret->status == BLKIF_RSP_OKAY),
+                       ret = end_that_request_first(req, uptodate,
                                req->hard_nr_sectors);
                        BUG_ON(ret);
-                       end_that_request_last(
-                               req, (bret->status == BLKIF_RSP_OKAY));
+                       end_that_request_last(req, uptodate);
                        break;
                default:
                        BUG();
Index: build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkfront/block.h
===================================================================
--- 
build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkfront/block.h
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkfront/block.h
@@ -126,6 +126,7 @@ struct blkfront_info
        struct gnttab_free_callback callback;
        struct blk_shadow shadow[BLK_RING_SIZE];
        unsigned long shadow_free;
+       int feature_barrier;
 
        /**
         * The number of people holding this device open.  We won't allow a
@@ -152,5 +153,6 @@ extern void do_blkif_request (request_qu
 int xlvbd_add(blkif_sector_t capacity, int device,
              u16 vdisk_info, u16 sector_size, struct blkfront_info *info);
 void xlvbd_del(struct blkfront_info *info);
+int xlvbd_barrier(struct blkfront_info *info);
 
 #endif /* __XEN_DRIVERS_BLOCK_H__ */
Index: build-32-unstable-11407/xen/include/public/io/blkif.h
===================================================================
--- build-32-unstable-11407.orig/xen/include/public/io/blkif.h
+++ build-32-unstable-11407/xen/include/public/io/blkif.h
@@ -29,8 +29,9 @@
 #endif
 #define blkif_sector_t uint64_t
 
-#define BLKIF_OP_READ      0
-#define BLKIF_OP_WRITE     1
+#define BLKIF_OP_READ              0
+#define BLKIF_OP_WRITE             1
+#define BLKIF_OP_WRITE_BARRIER     2
 
 /*
  * Maximum scatter/gather segments per request.
@@ -61,8 +62,9 @@ struct blkif_response {
 };
 typedef struct blkif_response blkif_response_t;
 
-#define BLKIF_RSP_ERROR  -1 /* non-specific 'error' */
-#define BLKIF_RSP_OKAY    0 /* non-specific 'okay'  */
+#define BLKIF_RSP_EOPNOTSUPP  -2 /* operation not supported (can happen on 
barrier writes) */
+#define BLKIF_RSP_ERROR       -1 /* non-specific 'error' */
+#define BLKIF_RSP_OKAY         0 /* non-specific 'okay'  */
 
 /*
  * Generate blkif ring structures and types.
Index: build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkfront/vbd.c
===================================================================
--- build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkfront/vbd.c
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkfront/vbd.c
@@ -257,6 +257,10 @@ xlvbd_alloc_gendisk(int minor, blkif_sec
        }
 
        info->rq = gd->queue;
+       info->gd = gd;
+
+       if (info->feature_barrier)
+               xlvbd_barrier(info);
 
        if (vdisk_info & VDISK_READONLY)
                set_disk_ro(gd, 1);
@@ -267,8 +271,6 @@ xlvbd_alloc_gendisk(int minor, blkif_sec
        if (vdisk_info & VDISK_CDROM)
                gd->flags |= GENHD_FL_CD;
 
-       info->gd = gd;
-
        return 0;
 
  out:
@@ -316,3 +318,17 @@ xlvbd_del(struct blkfront_info *info)
        blk_cleanup_queue(info->rq);
        info->rq = NULL;
 }
+
+int
+xlvbd_barrier(struct blkfront_info *info)
+{
+       int err;
+
+       err = blk_queue_ordered(info->rq,
+               info->feature_barrier ? QUEUE_ORDERED_DRAIN : 
QUEUE_ORDERED_NONE, NULL);
+       if (err)
+               return err;
+       printk("blkfront: %s: barriers %s\n",
+              info->gd->disk_name, info->feature_barrier ? "enabled" : 
"disabled");
+       return 0;
+}
Index: build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/vbd.c
===================================================================
--- build-32-unstable-11407.orig/linux-2.6-xen-sparse/drivers/xen/blkback/vbd.c
+++ build-32-unstable-11407/linux-2.6-xen-sparse/drivers/xen/blkback/vbd.c
@@ -104,7 +104,7 @@ int vbd_translate(struct phys_req *req, 
        struct vbd *vbd = &blkif->vbd;
        int rc = -EACCES;
 
-       if ((operation == WRITE) && vbd->readonly)
+       if ((operation != READ) && vbd->readonly)
                goto out;
 
        if (unlikely((req->sector_number + req->nr_sects) > vbd_sz(vbd)))
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