linux-kernel - Re: [PATCH v4 2/3] btrfs: Split remaining space to discard in chunks

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Message-ID: <cf8647d2-aa92-466d-8f79-494f9bd3e114@gmail.com>
Date: Mon, 16 Sep 2024 12:51:14 +0200
From: Luca Stefani <luca.stefani.ge1@...il.com>
To: Qu Wenruo <wqu@...e.com>
Cc: Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
 David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/3] btrfs: Split remaining space to discard in chunks



On 16/09/24 12:39, Qu Wenruo wrote:
> 
> 
> 在 2024/9/16 19:46, Luca Stefani 写道:
>> Per Qu Wenruo in case we have a very large disk, e.g. 8TiB device,
>> mostly empty although we will do the split according to our super block
>> locations, the last super block ends at 256G, we can submit a huge
>> discard for the range [256G, 8T), causing a super large delay.
>>
>> We now split the space left to discard based on BTRFS_MAX_DATA_CHUNK_SIZE
>> in preparation of introduction of cancellation signals handling.
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=219180
>> Link: https://bugzilla.suse.com/show_bug.cgi?id=1229737
>> Signed-off-by: Luca Stefani <luca.stefani.ge1@...il.com>
>> ---
>>   fs/btrfs/extent-tree.c | 24 +++++++++++++++++++-----
>>   1 file changed, 19 insertions(+), 5 deletions(-)
>>
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index a5966324607d..cbe66d0acff8 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -1239,7 +1239,7 @@ static int btrfs_issue_discard(struct 
>> block_device *bdev, u64 start, u64 len,
>>                      u64 *discarded_bytes)
>>   {
>>       int j, ret = 0;
>> -    u64 bytes_left, end;
>> +    u64 bytes_left, bytes_to_discard, end;
>>       u64 aligned_start = ALIGN(start, 1 << SECTOR_SHIFT);
>>       /* Adjust the range to be aligned to 512B sectors if necessary. */
>> @@ -1300,13 +1300,27 @@ static int btrfs_issue_discard(struct 
>> block_device *bdev, u64 start, u64 len,
>>           bytes_left = end - start;
>>       }
>> -    if (bytes_left) {
>> +    while (bytes_left) {
>> +        if (bytes_left > BTRFS_MAX_DATA_CHUNK_SIZE)
>> +            bytes_to_discard = BTRFS_MAX_DATA_CHUNK_SIZE;
> 
> That MAX_DATA_CHUNK_SIZE is only possible for RAID0/RAID10/RAID5/RAID6, 
> by spanning the device extents across multiple devices.
> 
> For each device, the maximum size is limited to 1G (check 
> init_alloc_chunk_ctl_policy_regular()).
> 
> So you can just limit it to 1G instead.
> (If you want, you can also extract that into a macro as a cleanup).
I think SZ_1G is enough for now.
> 
> Furthermore, you can use min() instead of a if ().
> 
> So you only need:
> 
>          bytes_to_discard = min(SZ_1G, bytes_left);
> 
> Otherwise this looks good enough to me.
> If the 1G size is not good enough, we can later tune it to smaller values.
> 
> Personally speaking I think 1G would be enough.
> 
> Thanks,
> Qu
Ack, done in v5
>> +        else
>> +            bytes_to_discard = bytes_left;
>> +
>>           ret = blkdev_issue_discard(bdev, start >> SECTOR_SHIFT,
>> -                       bytes_left >> SECTOR_SHIFT,
>> +                       bytes_to_discard >> SECTOR_SHIFT,
>>                          GFP_NOFS);
>> -        if (!ret)
>> -            *discarded_bytes += bytes_left;
>> +
>> +        if (ret) {
>> +            if (ret != -EOPNOTSUPP)
>> +                break;
>> +            continue;
>> +        }
>> +
>> +        start += bytes_to_discard;
>> +        bytes_left -= bytes_to_discard;
>> +        *discarded_bytes += bytes_to_discard;
>>       }
>> +
>>       return ret;
>>   }