Re: merging discard request in the block layer

[Christoph Hellwig <hch@infradead.org>]

I finally maged to debug this a bit further and can have found a cure
for the null pointer derefences I got on recent kernels.

The problem is that bio_has_data thinks discard requests have a payload
and thus tries to poke into it's pages when trying to merge requests.
Taking the REQ_DISCARD check in to bio_has_data fixes that.  I've also
tried to special case discard requests in bio_cur_bytes, but that
doesn't fix the botched requests messages yet.  I suspect the merging
code might need some additions to update the bio_size for discard
requests that it currently skips.

Index: xfs/block/blk-core.c
===================================================================
--- xfs.orig/block/blk-core.c	2011-05-03 19:45:51.980219652 +0200
+++ xfs/block/blk-core.c	2011-05-03 19:47:51.756237436 +0200
@@ -1645,7 +1645,7 @@ void submit_bio(int rw, struct bio *bio)
 	 * If it's a regular read/write or a barrier with data attached,
 	 * go through the normal accounting stuff before submission.
 	 */
-	if (bio_has_data(bio) && !(rw & REQ_DISCARD)) {
+	if (bio_has_data(bio)) {
 		if (rw & WRITE) {
 			count_vm_events(PGPGOUT, count);
 		} else {
Index: xfs/include/linux/bio.h
===================================================================
--- xfs.orig/include/linux/bio.h	2011-05-03 19:43:28.537663414 +0200
+++ xfs/include/linux/bio.h	2011-05-03 19:48:18.632758500 +0200
@@ -69,7 +69,7 @@
 
 static inline unsigned int bio_cur_bytes(struct bio *bio)
 {
-	if (bio->bi_vcnt)
+	if (bio->bi_vcnt && !(bio->bi_rw & REQ_DISCARD))
 		return bio_iovec(bio)->bv_len;
 	else /* dataless requests such as discard */
 		return bio->bi_size;
@@ -368,7 +368,7 @@ static inline char *__bio_kmap_irq(struc
  */
 static inline int bio_has_data(struct bio *bio)
 {
-	return bio && bio->bi_io_vec != NULL;
+	return bio && bio->bi_io_vec != NULL && !(bio->bi_rw & REQ_DISCARD);
 }
 
 /*