Re: [PATCH] btrfs: avoid defragging extents whose next extents are not targets

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

On 2022/3/15 19:15, Filipe Manana wrote:
> On Tue, Mar 15, 2022 at 09:07:52AM +0800, Qu Wenruo wrote:
>> [BUG]
>> There is a report that autodefrag is defragging single sector, which
>> is completely waste of IO, and no help for defragging:
>>
>>     btrfs-cleaner-808     defrag_one_locked_range: root=256 ino=651122 start=0 len=4096
>>
>> [CAUSE]
>> In defrag_collect_targets(), we check if the current range (A) can be merged
>> with next one (B).
>>
>> If mergeable, we will add range A into target for defrag.
>>
>> However there is a catch for autodefrag, when checking mergebility against
>> range B, we intentionally pass 0 as @newer_than, hoping to get a
>> higher chance to merge with the next extent.
>>
>> But in next iteartion, range B will looked up by defrag_lookup_extent(),
>> with non-zero @newer_than.
>>
>> And if range B is not really newer, it will rejected directly, causing
>> only range A being defragged, while we expect to defrag both range A and
>> B.
>>
>> [FIX]
>> Since the root cause is the difference in check condition of
>> defrag_check_next_extent() and defrag_collect_targets(), we fix it by:
>>
>> 1. Pass @newer_than to defrag_check_next_extent()
>> 2. Pass @extent_thresh to defrag_check_next_extent()
>>
>> This makes the check between defrag_collect_targets() and
>> defrag_check_next_extent() more consistent.
>>
>> While there is still some minor difference, the remaining checks are
>> focus on runtime flags like writeback/delalloc, which are mostly
>> transient and safe to be checked only in defrag_collect_targets().
>>
>> Issue: 423#issuecomment-1066981856
>
> Where is the issue exactly? It's the first time I'm seeing an Issue tag
> for kernel patches. Is this a github issue? If so, which repo? There are
> several repos we use for btrfs:
>
> https://github.com/btrfs/linux
> https://github.com/kdave/btrfs-devel

It's the btrfs/linux repo.

So I guess the proper way to link this is using Link: tag instead.

The full URL for it:
https://github.com/btrfs/linux/issues/423#issuecomment-1066981856

> https://github.com/kdave/btrfs-progs
> https://github.com/btrfs/fstests
> etc
>
> Can't we use a Link tag with an URL? That removes any doubts where the
> issue is and makes it easier to look at it.
>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>
> Doesn't this miss a Fixes tag and a CC stable tag for 5.16?

Oh, forgot to add that.

Will update the commit message for both the proper Link: and Cc: tag.

Thanks,
Qu
>
> This is fixing code added in 5.16, and given that users are reporting
> autodefrag causing disruptive amounts of IO, I don't see why it doesn't
> have a CC tag for stable.
>
> The change itself looks good. Thanks.
>
> Reviewed-by: Filipe Manana <fdmanana@suse.com>
>
>> ---
>>   fs/btrfs/ioctl.c | 21 +++++++++++++++------
>>   1 file changed, 15 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
>> index 3d3d6e2f110a..7d7520a2e281 100644
>> --- a/fs/btrfs/ioctl.c
>> +++ b/fs/btrfs/ioctl.c
>> @@ -1189,7 +1189,7 @@ static u32 get_extent_max_capacity(const struct extent_map *em)
>>   }
>>
>>   static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
>> -				     bool locked)
>> +				     u32 extent_thresh, u64 newer_than, bool locked)
>>   {
>>   	struct extent_map *next;
>>   	bool ret = false;
>> @@ -1199,11 +1199,13 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
>>   		return false;
>>
>>   	/*
>> -	 * We want to check if the next extent can be merged with the current
>> -	 * one, which can be an extent created in a past generation, so we pass
>> -	 * a minimum generation of 0 to defrag_lookup_extent().
>> +	 * Here we need to pass @newer_then when checking the next extent, or
>> +	 * we will hit a case we mark current extent for defrag, but the next
>> +	 * one will not be a target.
>> +	 * This will just cause extra IO without really reducing the fragments.
>>   	 */
>> -	next = defrag_lookup_extent(inode, em->start + em->len, 0, locked);
>> +	next = defrag_lookup_extent(inode, em->start + em->len, newer_than,
>> +				    locked);
>>   	/* No more em or hole */
>>   	if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE)
>>   		goto out;
>> @@ -1215,6 +1217,13 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
>>   	 */
>>   	if (next->len >= get_extent_max_capacity(em))
>>   		goto out;
>> +	/* Skip older extent */
>> +	if (next->generation < newer_than)
>> +		goto out;
>> +	/* Also check extent size */
>> +	if (next->len >= extent_thresh)
>> +		goto out;
>> +
>>   	ret = true;
>>   out:
>>   	free_extent_map(next);
>> @@ -1420,7 +1429,7 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
>>   			goto next;
>>
>>   		next_mergeable = defrag_check_next_extent(&inode->vfs_inode, em,
>> -							  locked);
>> +						extent_thresh, newer_than, locked);
>>   		if (!next_mergeable) {
>>   			struct defrag_target_range *last;
>>
>> --
>> 2.35.1
>>