block: reduce stack footprint of blk_recount_segments()

blk_recalc_rq_segments() requires a request structure passed in, which
we don't have from blk_recount_segments(). So the latter allocates one on
the stack, using > 400 bytes of stack for that. This can cause us to spill
over one page of stack from ext4 at least:

 0)     4560     400   blk_recount_segments+0x43/0x62
 1)     4160      32   bio_phys_segments+0x1c/0x24
 2)     4128      32   blk_rq_bio_prep+0x2a/0xf9
 3)     4096      32   init_request_from_bio+0xf9/0xfe
 4)     4064     112   __make_request+0x33c/0x3f6
 5)     3952     144   generic_make_request+0x2d1/0x321
 6)     3808      64   submit_bio+0xb9/0xc3
 7)     3744      48   submit_bh+0xea/0x10e
 8)     3696     368   ext4_mb_init_cache+0x257/0xa6a [ext4]
 9)     3328     288   ext4_mb_regular_allocator+0x421/0xcd9 [ext4]
10)     3040     160   ext4_mb_new_blocks+0x211/0x4b4 [ext4]
11)     2880     336   ext4_ext_get_blocks+0xb61/0xd45 [ext4]
12)     2544      96   ext4_get_blocks_wrap+0xf2/0x200 [ext4]
13)     2448      80   ext4_da_get_block_write+0x6e/0x16b [ext4]
14)     2368     352   mpage_da_map_blocks+0x7e/0x4b3 [ext4]
15)     2016     352   ext4_da_writepages+0x2ce/0x43c [ext4]
16)     1664      32   do_writepages+0x2d/0x3c
17)     1632     144   __writeback_single_inode+0x162/0x2cd
18)     1488      96   generic_sync_sb_inodes+0x1e3/0x32b
19)     1392      16   sync_sb_inodes+0xe/0x10
20)     1376      48   writeback_inodes+0x69/0xb3
21)     1328     208   balance_dirty_pages_ratelimited_nr+0x187/0x2f9
22)     1120     224   generic_file_buffered_write+0x1d4/0x2c4
23)      896     176   __generic_file_aio_write_nolock+0x35f/0x393
24)      720      80   generic_file_aio_write+0x6c/0xc8
25)      640      80   ext4_file_write+0xa9/0x137 [ext4]
26)      560     320   do_sync_write+0xf0/0x137
27)      240      48   vfs_write+0xb3/0x13c
28)      192      64   sys_write+0x4c/0x74
29)      128     128   system_call_fastpath+0x16/0x1b

Split the segment counting out into a __blk_recalc_rq_segments() helper
to avoid allocating an onstack request just for checking the physical
segment count.
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/blk-merge.c

@@ -38,28 +38,29 @@ void blk_recalc_rq_sectors(struct request *rq, int nsect)
 	}
 }
 
-void blk_recalc_rq_segments(struct request *rq)
+static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
+					     struct bio *bio,
+					     unsigned int *seg_size_ptr)
 {
-	int nr_phys_segs;
 	unsigned int phys_size;
 	struct bio_vec *bv, *bvprv = NULL;
-	int seg_size;
-	int cluster;
-	struct req_iterator iter;
-	int high, highprv = 1;
-	struct request_queue *q = rq->q;
+	int cluster, i, high, highprv = 1;
+	unsigned int seg_size, nr_phys_segs;
+	struct bio *fbio;
 
-	if (!rq->bio)
-		return;
+	if (!bio)
+		return 0;
 
+	fbio = bio;
 	cluster = test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
 	seg_size = 0;
 	phys_size = nr_phys_segs = 0;
-	rq_for_each_segment(bv, rq, iter) {
+	for_each_bio(bio) {
+		bio_for_each_segment(bv, bio, i) {
 			/*
-		 * the trick here is making sure that a high page is never
-		 * considered part of another segment, since that might
-		 * change with the bounce page.
+			 * the trick here is making sure that a high page is
+			 * never considered part of another segment, since that
+			 * might change with the bounce page.
 			 */
 			high = page_to_pfn(bv->bv_page) > q->bounce_pfn;
 			if (high || highprv)
@@ -77,33 +78,44 @@ void blk_recalc_rq_segments(struct request *rq)
 				continue;
 			}
 new_segment:
-		if (nr_phys_segs == 1 && seg_size > rq->bio->bi_seg_front_size)
-			rq->bio->bi_seg_front_size = seg_size;
+			if (nr_phys_segs == 1 && seg_size >
+			    fbio->bi_seg_front_size)
+				fbio->bi_seg_front_size = seg_size;
 
 			nr_phys_segs++;
 			bvprv = bv;
 			seg_size = bv->bv_len;
 			highprv = high;
 		}
+	}
+
+	if (seg_size_ptr)
+		*seg_size_ptr = seg_size;
 
-	if (nr_phys_segs == 1 && seg_size > rq->bio->bi_seg_front_size)
+	return nr_phys_segs;
+}
+
+void blk_recalc_rq_segments(struct request *rq)
+{
+	unsigned int seg_size = 0, phys_segs;
+
+	phys_segs = __blk_recalc_rq_segments(rq->q, rq->bio, &seg_size);
+
+	if (phys_segs == 1 && seg_size > rq->bio->bi_seg_front_size)
 		rq->bio->bi_seg_front_size = seg_size;
 	if (seg_size > rq->biotail->bi_seg_back_size)
 		rq->biotail->bi_seg_back_size = seg_size;
 
-	rq->nr_phys_segments = nr_phys_segs;
+	rq->nr_phys_segments = phys_segs;
 }
 
 void blk_recount_segments(struct request_queue *q, struct bio *bio)
 {
-	struct request rq;
 	struct bio *nxt = bio->bi_next;
-	rq.q = q;
-	rq.bio = rq.biotail = bio;
+
 	bio->bi_next = NULL;
-	blk_recalc_rq_segments(&rq);
+	bio->bi_phys_segments = __blk_recalc_rq_segments(q, bio, NULL);
 	bio->bi_next = nxt;
-	bio->bi_phys_segments = rq.nr_phys_segments;
 	bio->bi_flags |= (1 << BIO_SEG_VALID);
 }
 EXPORT_SYMBOL(blk_recount_segments);

include/linux/blkdev.h

@@ -708,6 +708,8 @@ struct req_iterator {
 };
 
 /* This should not be used directly - use rq_for_each_segment */
+#define for_each_bio(_bio)		\
+	for (; _bio; _bio = _bio->bi_next)
 #define __rq_for_each_bio(_bio, rq)	\
 	if ((rq->bio))			\
 		for (_bio = (rq)->bio; _bio; _bio = _bio->bi_next)