[dm-devel] [PATCH] dm-io: don't allocate vector larger than BIO_MAX_PAGES

[Mikulas Patocka]

dm-io calls bio_get_nr_vecs to get the maximum number of pages for a given
device, then adds 1 to it and allocates bio with this size.

The last vector is not used for i/o, it is used to hold information about
the region this i/o belongs to.

If bio_get_nr_vecs returned the maximum biovec size, dm-io attempts to allocate
bio with one more vector and fails.

Very likely this was the reason for bug
https://bugzilla.redhat.com/show_bug.cgi?id=173153

(the bug was fixed with an userspace workaround preventing lvm from creating
snapshots with chunksize >512k; after this patch is applied, that limit can
be dropped)

Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>

---
 drivers/md/dm-io.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6.29-rc1-devel/drivers/md/dm-io.c
===================================================================
--- linux-2.6.29-rc1-devel.orig/drivers/md/dm-io.c	2009-01-22 04:13:45.000000000 +0100
+++ linux-2.6.29-rc1-devel/drivers/md/dm-io.c	2009-01-22 04:14:13.000000000 +0100
@@ -292,6 +292,8 @@ static void do_region(int rw, unsigned r
 					     (PAGE_SIZE >> SECTOR_SHIFT));
 		num_bvecs = 1 + min_t(int, bio_get_nr_vecs(where->bdev),
 				      num_bvecs);
+		if (unlikely(num_bvecs > BIO_MAX_PAGES))
+			num_bvecs = BIO_MAX_PAGES;
 		bio = bio_alloc_bioset(GFP_NOIO, num_bvecs, io->client->bios);
 		bio->bi_sector = where->sector + (where->count - remaining);
 		bio->bi_bdev = where->bdev;