Samuel Thibault, le Thu 07 Feb 2008 16:14:42 +0000, a écrit :
> Nothing is said about parameters to be given along WRITE_BARRIER or
> FLUSH_DISKCACHE operations. I guess they are both implicitely supposed
> to be write operations? (it's the case for WRITE_BARRIER in the Linux
> implementation) If so, would it be fine to allow nr_segments to be 0?
> As part of IDE cache flush emulation, I need to issue WRITE_BARRIERs
> without issuing any actual write...
The patch below implements it for Linux:
block: backport Jens Axboe's commit from
Tue, 16 Oct 2007 09:03:56 +0000 (11:03 +0200)
bf2de6f5a4faf0197268f18d08969b003b87b6e8
Initial support for data-less (or empty) barrier support
blkback: permit and implement empty barrier.
Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxxx>
diff -r 5c61cd349b20 block/elevator.c
--- a/block/elevator.c Thu Feb 07 10:33:19 2008 +0000
+++ b/block/elevator.c Fri Feb 08 11:27:12 2008 +0000
@@ -493,6 +493,16 @@
int ret;
while ((rq = __elv_next_request(q)) != NULL) {
+ /*
+ * Kill the empty barrier place holder, the driver must
+ * not ever see it.
+ */
+ if (blk_empty_barrier(rq)) {
+ blkdev_dequeue_request(rq);
+ end_that_request_chunk(rq, 1, 0);
+ end_that_request_last(rq, 1);
+ continue;
+ }
if (!(rq->flags & REQ_STARTED)) {
elevator_t *e = q->elevator;
--- a/block/ll_rw_blk.c Thu Feb 07 10:33:19 2008 +0000
+++ b/block/ll_rw_blk.c Fri Feb 08 11:27:12 2008 +0000
@@ -483,9 +483,12 @@
* Queue ordered sequence. As we stack them at the head, we
* need to queue in reverse order. Note that we rely on that
* no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
- * request gets inbetween ordered sequence.
+ * request gets inbetween ordered sequence. If this request is
+ * an empty barrier, we don't need to do a postflush ever since
+ * there will be no data written between the pre and post flush.
+ * Hence a single flush will suffice.
*/
- if (q->ordered & QUEUE_ORDERED_POSTFLUSH)
+ if ((q->ordered & QUEUE_ORDERED_POSTFLUSH) && !blk_empty_barrier(rq))
queue_flush(q, QUEUE_ORDERED_POSTFLUSH);
else
q->ordseq |= QUEUE_ORDSEQ_POSTFLUSH;
@@ -2967,7 +2970,7 @@
{
struct block_device *bdev = bio->bi_bdev;
- if (bdev != bdev->bd_contains) {
+ if (bio_sectors(bio) && bdev != bdev->bd_contains) {
struct hd_struct *p = bdev->bd_part;
const int rw = bio_data_dir(bio);
@@ -3028,7 +3031,7 @@
might_sleep();
/* Test device or partition size, when known. */
maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
- if (maxsector) {
+ if (maxsector && nr_sectors) {
sector_t sector = bio->bi_sector;
if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
@@ -3094,7 +3097,7 @@
old_dev = bio->bi_bdev->bd_dev;
maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
- if (maxsector) {
+ if (maxsector && nr_sectors) {
sector_t sector = bio->bi_sector;
if (maxsector < nr_sectors || maxsector - nr_sectors <
sector) {
@@ -3128,21 +3131,25 @@
{
int count = bio_sectors(bio);
- BIO_BUG_ON(!bio->bi_size);
- BIO_BUG_ON(!bio->bi_io_vec);
bio->bi_rw |= rw;
- if (rw & WRITE)
- count_vm_events(PGPGOUT, count);
- else
- count_vm_events(PGPGIN, count);
- if (unlikely(block_dump)) {
- char b[BDEVNAME_SIZE];
- printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
- current->comm, current->pid,
- (rw & WRITE) ? "WRITE" : "READ",
- (unsigned long long)bio->bi_sector,
- bdevname(bio->bi_bdev,b));
+ if (!bio_empty_barrier(bio)) {
+ BIO_BUG_ON(!bio->bi_size);
+ BIO_BUG_ON(!bio->bi_io_vec);
+
+ if (rw & WRITE)
+ count_vm_events(PGPGOUT, count);
+ else
+ count_vm_events(PGPGIN, count);
+
+ if (unlikely(block_dump)) {
+ char b[BDEVNAME_SIZE];
+ printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
+ current->comm, current->pid,
+ (rw & WRITE) ? "WRITE" : "READ",
+ (unsigned long long)bio->bi_sector,
+ bdevname(bio->bi_bdev,b));
+ }
}
generic_make_request(bio);
@@ -3259,6 +3266,13 @@
total_bytes = bio_nbytes = 0;
while ((bio = req->bio) != NULL) {
int nbytes;
+
+ /* For an empty barrier request, the low level driver must
+ * store a potential error location in ->sector. We pass
+ * that back up in ->bi_sector
+ */
+ if (blk_empty_barrier(req))
+ bio->bi_sector = req->sector;
if (nr_bytes >= bio->bi_size) {
req->bio = bio->bi_next;
--- a/drivers/xen/blkback/blkback.c Thu Feb 07 10:33:19 2008 +0000
+++ b/drivers/xen/blkback/blkback.c Fri Feb 08 11:27:13 2008 +0000
@@ -407,7 +407,7 @@
/* Check that number of segments is sane. */
nseg = req->nr_segments;
- if (unlikely(nseg == 0) ||
+ if (unlikely(nseg == 0 && operation != WRITE_BARRIER) ||
unlikely(nseg > BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
DPRINTK("Bad number of segments in request (%d)\n", nseg);
goto fail_response;
@@ -500,6 +500,18 @@
preq.sector_number += seg[i].nsec;
}
+ if (!bio) {
+ BUG_ON(operation != WRITE_BARRIER);
+ bio = biolist[nbio++] = bio_alloc(GFP_KERNEL, 0);
+ if (unlikely(bio == NULL))
+ goto fail_put_bio;
+
+ bio->bi_bdev = preq.bdev;
+ bio->bi_private = pending_req;
+ bio->bi_end_io = end_block_io_op;
+ bio->bi_sector = -1;
+ }
+
plug_queue(blkif, bio);
atomic_set(&pending_req->pendcnt, nbio);
blkif_get(blkif);
--- a/fs/bio.c Thu Feb 07 10:33:19 2008 +0000
+++ b/fs/bio.c Fri Feb 08 11:27:13 2008 +0000
@@ -112,7 +112,8 @@
BIO_BUG_ON(pool_idx >= BIOVEC_NR_POOLS);
- mempool_free(bio->bi_io_vec, bio_set->bvec_pools[pool_idx]);
+ if (bio->bi_io_vec)
+ mempool_free(bio->bi_io_vec, bio_set->bvec_pools[pool_idx]);
mempool_free(bio, bio_set->bio_pool);
}
--- a/include/linux/bio.h Thu Feb 07 10:33:19 2008 +0000
+++ b/include/linux/bio.h Fri Feb 08 11:27:13 2008 +0000
@@ -172,12 +172,27 @@
#define bio_offset(bio) bio_iovec((bio))->bv_offset
#define bio_segments(bio) ((bio)->bi_vcnt - (bio)->bi_idx)
#define bio_sectors(bio) ((bio)->bi_size >> 9)
-#define bio_cur_sectors(bio) (bio_iovec(bio)->bv_len >> 9)
-#define bio_data(bio) (page_address(bio_page((bio))) +
bio_offset((bio)))
#define bio_barrier(bio) ((bio)->bi_rw & (1 << BIO_RW_BARRIER))
#define bio_sync(bio) ((bio)->bi_rw & (1 << BIO_RW_SYNC))
#define bio_failfast(bio) ((bio)->bi_rw & (1 << BIO_RW_FAILFAST))
#define bio_rw_ahead(bio) ((bio)->bi_rw & (1 << BIO_RW_AHEAD))
+#define bio_empty_barrier(bio) (bio_barrier(bio) && !(bio)->bi_size)
+
+static inline unsigned int bio_cur_sectors(struct bio *bio)
+{
+ if (bio->bi_vcnt)
+ return bio_iovec(bio)->bv_len >> 9;
+
+ return 0;
+}
+
+static inline void *bio_data(struct bio *bio)
+{
+ if (bio->bi_vcnt)
+ return page_address(bio_page(bio)) + bio_offset(bio);
+
+ return NULL;
+}
/*
* will die
--- a/include/linux/blkdev.h Thu Feb 07 10:33:19 2008 +0000
+++ b/include/linux/blkdev.h Fri Feb 08 11:27:13 2008 +0000
@@ -506,6 +506,8 @@
#define blk_barrier_rq(rq) ((rq)->flags & REQ_HARDBARRIER)
#define blk_fua_rq(rq) ((rq)->flags & REQ_FUA)
+#define blk_empty_barrier(rq) (blk_barrier_rq(rq) && blk_fs_request(rq) &&
!(rq)->hard_nr_sectors)
+
#define list_entry_rq(ptr) list_entry((ptr), struct request, queuelist)
#define rq_data_dir(rq) ((rq)->flags & 1)
--- a/mm/highmem.c Thu Feb 07 10:33:19 2008 +0000
+++ b/mm/highmem.c Fri Feb 08 11:27:13 2008 +0000
@@ -468,6 +468,12 @@
mempool_t *pool;
/*
+ * Data-less bio, nothing to bounce
+ */
+ if (bio_empty_barrier(*bio_orig))
+ return;
+
+ /*
* for non-isa bounce case, just check if the bounce pfn is equal
* to or bigger than the highest pfn in the system -- in that case,
* don't waste time iterating over bio segments
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