Re: [PATCH v2] btrfs: replace BUG_ON() in btrfs_csum_one_bio() with proper error handling

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2021/8/17 下午3:55, Nikolay Borisov wrote:
>
>
> On 17.08.21 г. 2:55, Qu Wenruo wrote:
>> There is a BUG_ON() in btrfs_csum_one_bio() to catch code logic error.
>>
>> It has indeed caught several bugs during subpage development.
>>
>> But the BUG_ON() itself will bring down the whole system which is
>> sometimes overkilled.
>>
>> Replace it with a WARN() and exit gracefully, so that it won't crash the
>> whole system while we can still catch the code logic error.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>> Changelog:
>> v2:
>> - Re-send as an independent patch
>> - Add WARN() to catch the code logic error
>> ---
>>   fs/btrfs/file-item.c | 13 ++++++++++++-
>>   1 file changed, 12 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
>> index 2673c6ba7a4e..7f58d80a480f 100644
>> --- a/fs/btrfs/file-item.c
>> +++ b/fs/btrfs/file-item.c
>> @@ -665,7 +665,18 @@ blk_status_t btrfs_csum_one_bio(struct btrfs_inode *inode, struct bio *bio,
>>
>>   		if (!ordered) {
>>   			ordered = btrfs_lookup_ordered_extent(inode, offset);
>> -			BUG_ON(!ordered); /* Logic error */
>> +			/*
>> +			 * The bio range is not covered by any ordered extent,
>> +			 * must be a code logic error.
>> +			 */
>> +			if (unlikely(!ordered)) {
>> +				WARN(1, KERN_WARNING
>> +		"no ordered extent for root %llu ino %llu offset %llu\n",
>> +				     inode->root->root_key.objectid,
>> +				     btrfs_ino(inode), offset);
>> +				kvfree(sums);
>> +				return BLK_STS_IOERR;
>> +			}
>
> nit: How about :
>
> if (WARN_ON(!ordered)  {

I still remember that if (WARN_ON()) usage is not recommended by David.

Is that still the case?

> btrfs_err(foo)
> }
>
> That way you get the unlikely(!ordered) 'for free' and the code is
> somewhat cleaner IMO.
>
> While at it I also have to say the structure of the inner loop is rather
> iffy, because it's really if/else with an implicit 'else'. How about
> converting it to https://paste.ubuntu.com/p/kyWsRrkzWq/

I have no obvious preference between the existing one and the new one.

But since you mentioned it, I would prefer a third way, exactly the code
of the existing if () branch into a function, like
switch_ordered_extent(), and add a comment before the if () line.

To me, that would look easier to read.

Thanks,
Qu
>
>>   		}
>>
>>   		nr_sectors = BTRFS_BYTES_TO_BLKS(fs_info,
>>