Re: [PATCH] ext4: fix dirty pages writback regression.

["Yan, Zheng" <zheng.z.yan@intel.com>]

On 09/10/2013 05:00 PM, Jan Kara wrote:
> On Tue 10-09-13 10:02:58, Yan, Zheng wrote:
>> From: "Yan, Zheng" <zheng.z.yan@intel.com>
>>
>> Our Linux Kernel Performance project found that commit 4e7ea81db5
>> (ext4: restructure writeback path) indroduced regression. After
>> the commit, ext4 does not merge adjacent mapped dirty pages during
>> writeback. The "!buffer_delay(bh) && !buffer_unwritten(bh)" check
>> in mpage_add_bh_to_extent() prevents the merging.
>>
>> Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
>> ---
>>  fs/ext4/inode.c | 3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index c79fd7d..bfeb8b2 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -1944,8 +1944,7 @@ static bool mpage_add_bh_to_extent(struct mpage_da_data *mpd, ext4_lblk_t lblk,
>>  	struct ext4_map_blocks *map = &mpd->map;
>>  
>>  	/* Buffer that doesn't need mapping for writeback? */
>> -	if (!buffer_dirty(bh) || !buffer_mapped(bh) ||
>> -	    (!buffer_delay(bh) && !buffer_unwritten(bh))) {
>> +	if (!buffer_dirty(bh) || !buffer_mapped(bh)) {
>   Sadly it isn't that easy. The condition is there for a reason... The
> reason is that we are looking for an extent to map. When we already have
> some buffer to map and then there is buffer which doesn't need mapping we
> cannot just add it to the extent because then we would allocate too many
> blocks. 

the "(b_state & BH_FLAGS) == map->m_flags)" check in mpage_add_bh_to_extent() should
prevent delayed and non-delayed dirty pages from merging. What am I missing here?

Regards
Yan, Zheng

> Also the transaction credits we have reserved are just for
> allocation of one extent and its possible conversion from unwritten to
> written extent. So that's another reason why you cannot arbitrarily merge
> allocated and unallocated buffers or written and unwritten buffers.
> 
> Now also I'm somewhat surprised that this condition is causing a regression
> because it was also present in the previous version of the code although it
> was there in a different place and in a slightly different form. I'll try to
> reproduce results using your fio script and will have a look at what is
> causing the problem.
> 
> 								Honza
> 
>>  		/* So far no extent to map => we write the buffer right away */
>>  		if (map->m_len == 0)
>>  			return true;
>> -- 
>> 1.8.1.4
>>