linux-kernel - Re: [PATCH 1/2] btrfs: zoned: use rcu list for iterating devices to collect stats

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Date: Tue, 23 Jan 2024 07:49:22 +0000
From: Johannes Thumshirn <Johannes.Thumshirn@....com>
To: "dsterba@...e.cz" <dsterba@...e.cz>
CC: Josef Bacik <josef@...icpanda.com>, David Sterba <dsterba@...e.com>,
	Naohiro Aota <Naohiro.Aota@....com>, "linux-btrfs@...r.kernel.org"
	<linux-btrfs@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, Damien Le Moal <dlemoal@...nel.org>
Subject: Re: [PATCH 1/2] btrfs: zoned: use rcu list for iterating devices to
 collect stats

On 22.01.24 22:35, David Sterba wrote:
> On Mon, Jan 22, 2024 at 02:51:03AM -0800, Johannes Thumshirn wrote:
>> As btrfs_zoned_should_reclaim only has to iterate the device list in order
>> to collect stats on the device's total and used bytes, we don't need to
>> take the full blown mutex, but can iterate the device list in a rcu_read
>> context.
>>
>> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@....com>
>> ---
>>   fs/btrfs/zoned.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
>> index 168af9d000d1..b7e7b5a5a6fa 100644
>> --- a/fs/btrfs/zoned.c
>> +++ b/fs/btrfs/zoned.c
>> @@ -2423,15 +2423,15 @@ bool btrfs_zoned_should_reclaim(struct btrfs_fs_info *fs_info)
>>   	if (fs_info->bg_reclaim_threshold == 0)
>>   		return false;
>>   
>> -	mutex_lock(&fs_devices->device_list_mutex);
>> -	list_for_each_entry(device, &fs_devices->devices, dev_list) {
>> +	rcu_read_lock();
>> +	list_for_each_entry_rcu(device, &fs_devices->devices, dev_list) {
>>   		if (!device->bdev)
>>   			continue;
>>   
>>   		total += device->disk_total_bytes;
>>   		used += device->bytes_used;
>>   	}
>> -	mutex_unlock(&fs_devices->device_list_mutex);
>> +	rcu_read_unlock();
> 
> This is basically only a hint and inaccuracies in the total or used
> values would be transient, right? The sum is calculated each time the
> funciton is called, not stored anywhere so in the unlikely case of
> device removal it may skip reclaim once, but then pick it up later.
> Any actual removal of the block groups in verified again and properly
> locked in btrfs_reclaim_bgs_work().
> 

Yes.