Re: [PATCH] block: make sure last_lookup set as NULL after part deleted

[Yufen Yu <yuyufen@huawei.com>]

On 2019/12/31 22:55, houtao (A) wrote:

> diff --git a/block/genhd.c b/block/genhd.c
> index 6e8543ca6912..179e0056fae1 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -279,7 +279,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
>   		part = rcu_dereference(ptbl->part[i]);
> 
>   		if (part && sector_in_part(part, sector)) {
> -			rcu_assign_pointer(ptbl->last_lookup, part);
> +			struct hd_struct *old;
> +
> +			if (!hd_struct_try_get(part))
> +				break;
> +
> +			old = xchg(&ptbl->last_lookup, part);
> +			if (old)
> +				hd_struct_put(old);
>   			return part;
>   		}
>   	}
> @@ -1231,7 +1238,11 @@ static void disk_replace_part_tbl(struct gendisk *disk,
>   	rcu_assign_pointer(disk->part_tbl, new_ptbl);
> 
>   	if (old_ptbl) {
> -		rcu_assign_pointer(old_ptbl->last_lookup, NULL);
> +		struct hd_struct *part;
> +
> +		part = xchg(&old_ptbl->last_lookup, NULL);
> +		if (part)
> +			hd_struct_put(part);
>   		kfree_rcu(old_ptbl, rcu_head);
>   	}
>   }
> diff --git a/block/partition-generic.c b/block/partition-generic.c
> index 98d60a59b843..441c1c591c04 100644
> --- a/block/partition-generic.c
> +++ b/block/partition-generic.c
> @@ -285,7 +285,8 @@ void delete_partition(struct gendisk *disk, int partno)
>   		return;
> 
>   	rcu_assign_pointer(ptbl->part[partno], NULL);
> -	rcu_assign_pointer(ptbl->last_lookup, NULL);
> +	if (cmpxchg(&ptbl->last_lookup, part, NULL) == part)
> +		hd_struct_put(part);
>   	kobject_put(part->holder_dir);
>   	device_del(part_to_dev(part));
> 
This change will add more calling of hd_struct_try_get()/hd_struct_put()
when set last_lookup. Thus, Tao warried abort extra overhead.

We test the patch on a SSD disk by fio with config:
[global]
rw=randread
bs=4k
numjobs=2
ioengine=libaio
iodepth=32
direct=1
runtime=120
size=100G
group_reporting
time_based

[sda1]
filename=/dev/sda1

[sda2]
filename=/dev/sda2

[sda3]
filename=/dev/sda3

[sda4]
filename=/dev/sda4

Compared with the previous version, io bandwidth have no any degeneration.

Thanks,
Yufen