blk-iocost: fix divide by 0 error in calc_lcoefs()

echo max of u64 to cost.model can cause divide by 0 error.

  # echo 8:0 rbps=18446744073709551615 > /sys/fs/cgroup/io.cost.model

  divide error: 0000 [#1] PREEMPT SMP
  RIP: 0010:calc_lcoefs+0x4c/0xc0
  Call Trace:
   <TASK>
   ioc_refresh_params+0x2b3/0x4f0
   ioc_cost_model_write+0x3cb/0x4c0
   ? _copy_from_iter+0x6d/0x6c0
   ? kernfs_fop_write_iter+0xfc/0x270
   cgroup_file_write+0xa0/0x200
   kernfs_fop_write_iter+0x17d/0x270
   vfs_write+0x414/0x620
   ksys_write+0x73/0x160
   __x64_sys_write+0x1e/0x30
   do_syscall_64+0x35/0x80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

calc_lcoefs() uses the input value of cost.model in DIV_ROUND_UP_ULL,
overflow would happen if bps plus IOC_PAGE_SIZE is greater than
ULLONG_MAX, it can cause divide by 0 error.

Fix the problem by setting basecost
Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230117070806.3857142-5-yukuai1@huaweicloud.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-iocost.c

@@ -868,9 +868,14 @@ static void calc_lcoefs(u64 bps, u64 seqiops, u64 randiops,
 
 	*page = *seqio = *randio = 0;
 
-	if (bps)
-		*page = DIV64_U64_ROUND_UP(VTIME_PER_SEC,
-					   DIV_ROUND_UP_ULL(bps, IOC_PAGE_SIZE));
+	if (bps) {
+		u64 bps_pages = DIV_ROUND_UP_ULL(bps, IOC_PAGE_SIZE);
+
+		if (bps_pages)
+			*page = DIV64_U64_ROUND_UP(VTIME_PER_SEC, bps_pages);
+		else
+			*page = 1;
+	}
 
 	if (seqiops) {
 		v = DIV64_U64_ROUND_UP(VTIME_PER_SEC, seqiops);