Re: [PATCH v2] btrfs: reduce chunk_map lookups in btrfs_map_block

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 13.08.24 12:57, Filipe Manana wrote:
> On Tue, Aug 13, 2024 at 11:49 AM Johannes Thumshirn <jth@kernel.org> wrote:
>>
>> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>>
>> Currently we're calling btrfs_num_copies() before btrfs_get_chunk_map() in
>> btrfs_map_block(). But btrfs_num_copies() itself does a chunk map lookup
>> to be able to calculate the number of copies.
>>
>> So split out the code getting the number of copies from btrfs_num_copies()
>> into a helper called btrfs_chunk_map_num_copies() and directly call it
>> from btrfs_map_block() and btrfs_num_copies().
>>
>> This saves us one rbtree lookup per btrfs_map_block() invocation.
>>
>> Reviewed-by: Qu Wenruo <wqu@suse.com>
>> Reviewed-by: Filipe Manana <fdmanana@suse.com>
>> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>> ---
>> Changes in v2:
>> - Added Reviewed-bys
>> - Reflowed comments
>> - Moved non RAID56 cases to the end without an if
>> Link to v1:
>> https://lore.kernel.org/all/20240812165931.9106-1-jth@kernel.org/
>>
>>   fs/btrfs/volumes.c | 58 +++++++++++++++++++++++++---------------------
>>   1 file changed, 32 insertions(+), 26 deletions(-)
>>
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index e07452207426..796f6350a017 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -5781,38 +5781,44 @@ void btrfs_mapping_tree_free(struct btrfs_fs_info *fs_info)
>>          write_unlock(&fs_info->mapping_tree_lock);
>>   }
>>
>> +static int btrfs_chunk_map_num_copies(struct btrfs_chunk_map *map)
> 
> Same as commented before, can be const.

No it can't:
fs/btrfs/volumes.c:5784:8: warning: type qualifiers ignored on function 
return type [-Wignored-qualifiers]
  5784 | static const int btrfs_chunk_map_num_copies(struct 
btrfs_chunk_map *map)

Or did you mean the const struct btrfs_chunk_map? That could be done but 
I don't see a reason.

> 
>> +{
>> +       enum btrfs_raid_types index = btrfs_bg_flags_to_raid_index(map->type);
>> +
>> +       if (map->type & BTRFS_BLOCK_GROUP_RAID5)
>> +               return 2;
>> +
>> +       /*
>> +        * There could be two corrupted data stripes, we need to loop
>> +        * retry in order to rebuild the correct data.
>> +        *
>> +        * Fail a stripe at a time on every retry except the stripe
>> +        * under reconstruction.
>> +        */
>> +       if (map->type & BTRFS_BLOCK_GROUP_RAID6)
>> +               return map->num_stripes;
>> +
>> +       /* Non-RAID56, use their ncopies from btrfs_raid_array. */
>> +       return btrfs_raid_array[index].ncopies;
>> +}
>> +
>>   int btrfs_num_copies(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
>>   {
>>          struct btrfs_chunk_map *map;
>> -       enum btrfs_raid_types index;
>> -       int ret = 1;
>> +       int ret;
>>
>>          map = btrfs_get_chunk_map(fs_info, logical, len);
>>          if (IS_ERR(map))
>>                  /*
>> -                * We could return errors for these cases, but that could get
>> -                * ugly and we'd probably do the same thing which is just not do
>> -                * anything else and exit, so return 1 so the callers don't try
>> -                * to use other copies.
>> +                * We could return errors for these cases, but that
>> +                * could get ugly and we'd probably do the same thing
>> +                * which is just not do anything else and exit, so
>> +                * return 1 so the callers don't try to use other
>> +                * copies.
> 
> My previous comment about reformatting was just for the comment that
> was moved, the one now inside btrfs_chunk_map_num_copies().
> For this one I don't think we should do it, as we aren't moving it
> around or changing its content.
> 
> It's just confusing to have this sort of unrelated change mixed in.
> 

Whoops that was fat fingered somehow.