block, bfq: Fix division by zero error on zero wsum

When the weighted sum is zero the calculation of limit causes
a division by zero error. Fix this by continuing to the next level.

This was discovered by running as root:

stress-ng --ioprio 0

Fixes divison by error oops:

[  521.450556] divide error: 0000 [#1] SMP NOPTI
[  521.450766] CPU: 2 PID: 2684464 Comm: stress-ng-iopri Not tainted 6.2.1-1280.native #1
[  521.451117] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.1-0-g3208b098f51a-prebuilt.qemu.org 04/01/2014
[  521.451627] RIP: 0010:bfqq_request_over_limit+0x207/0x400
[  521.451875] Code: 01 48 8d 0c c8 74 0b 48 8b 82 98 00 00 00 48 8d 0c c8 8b 85 34 ff ff ff 48 89 ca 41 0f af 41 50 48 d1 ea 48 98 48 01 d0 31 d2 <48> f7 f1 41 39 41 48 89 85 34 ff ff ff 0f 8c 7b 01 00 00 49 8b 44
[  521.452699] RSP: 0018:ffffb1af84eb3948 EFLAGS: 00010046
[  521.452938] RAX: 000000000000003c RBX: 0000000000000000 RCX: 0000000000000000
[  521.453262] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffb1af84eb3978
[  521.453584] RBP: ffffb1af84eb3a30 R08: 0000000000000001 R09: ffff8f88ab8a4ba0
[  521.453905] R10: 0000000000000000 R11: 0000000000000001 R12: ffff8f88ab8a4b18
[  521.454224] R13: ffff8f8699093000 R14: 0000000000000001 R15: ffffb1af84eb3970
[  521.454549] FS:  00005640b6b0b580(0000) GS:ffff8f88b3880000(0000) knlGS:0000000000000000
[  521.454912] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  521.455170] CR2: 00007ffcbcae4e38 CR3: 00000002e46de001 CR4: 0000000000770ee0
[  521.455491] PKRU: 55555554
[  521.455619] Call Trace:
[  521.455736]  <TASK>
[  521.455837]  ? bfq_request_merge+0x3a/0xc0
[  521.456027]  ? elv_merge+0x115/0x140
[  521.456191]  bfq_limit_depth+0xc8/0x240
[  521.456366]  __blk_mq_alloc_requests+0x21a/0x2c0
[  521.456577]  blk_mq_submit_bio+0x23c/0x6c0
[  521.456766]  __submit_bio+0xb8/0x140
[  521.457236]  submit_bio_noacct_nocheck+0x212/0x300
[  521.457748]  submit_bio_noacct+0x1a6/0x580
[  521.458220]  submit_bio+0x43/0x80
[  521.458660]  ext4_io_submit+0x23/0x80
[  521.459116]  ext4_do_writepages+0x40a/0xd00
[  521.459596]  ext4_writepages+0x65/0x100
[  521.460050]  do_writepages+0xb7/0x1c0
[  521.460492]  __filemap_fdatawrite_range+0xa6/0x100
[  521.460979]  file_write_and_wait_range+0xbf/0x140
[  521.461452]  ext4_sync_file+0x105/0x340
[  521.461882]  __x64_sys_fsync+0x67/0x100
[  521.462305]  ? syscall_exit_to_user_mode+0x2c/0x1c0
[  521.462768]  do_syscall_64+0x3b/0xc0
[  521.463165]  entry_SYSCALL_64_after_hwframe+0x5a/0xc4
[  521.463621] RIP: 0033:0x5640b6c56590
[  521.464006] Code: 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 80 3d 71 70 0e 00 00 74 17 b8 4a 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 48 c3 0f 1f 80 00 00 00 00 48 83 ec 18 89 7c
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20230413133009.1605335-1-colin.i.king@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/bfq-iosched.c

@@ -648,6 +648,8 @@ static bool bfqq_request_over_limit(struct bfq_queue *bfqq, int limit)
 					sched_data->service_tree[i].wsum;
 			}
 		}
+		if (!wsum)
+			continue;
 		limit = DIV_ROUND_CLOSEST(limit * entity->weight, wsum);
 		if (entity->allocated >= limit) {
 			bfq_log_bfqq(bfqq->bfqd, bfqq,