dm log writes: fix bug with too large bios

BugLink: http://bugs.launchpad.net/bugs/1631468

commit 7efb3673 upstream.

bio_alloc() can allocate a bio with at most BIO_MAX_PAGES (256) vector
entries.  However, the incoming bio may have more vector entries if it
was allocated by other means.  For example, bcache submits bios with
more than BIO_MAX_PAGES entries.  This results in bio_alloc() failure.

To avoid the failure, change the code so that it allocates bio with at
most BIO_MAX_PAGES entries.  If the incoming bio has more entries,
bio_add_page() will fail and a new bio will be allocated - the code that
handles bio_add_page() failure already exists in the dm-log-writes
target.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/md/dm-log-writes.c

@@ -259,7 +259,7 @@ static int log_one_block(struct log_writes_c *lc,
 	sector++;
 
 	atomic_inc(&lc->io_blocks);
-	bio = bio_alloc(GFP_KERNEL, block->vec_cnt);
+	bio = bio_alloc(GFP_KERNEL, min(block->vec_cnt, BIO_MAX_PAGES));
 	if (!bio) {
 		DMERR("Couldn't alloc log bio");
 		goto error;
@@ -280,7 +280,7 @@ static int log_one_block(struct log_writes_c *lc,
 		if (ret != block->vecs[i].bv_len) {
 			atomic_inc(&lc->io_blocks);
 			submit_bio(WRITE, bio);
-			bio = bio_alloc(GFP_KERNEL, block->vec_cnt - i);
+			bio = bio_alloc(GFP_KERNEL, min(block->vec_cnt - i, BIO_MAX_PAGES));
 			if (!bio) {
 				DMERR("Couldn't alloc log bio");
 				goto error;