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

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Date:	Tue, 10 Sep 2013 19:01:16 +0800
From:	"Yan, Zheng" <ukernel@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	"Yan, Zheng" <zheng.z.yan@...el.com>, linux-ext4@...r.kernel.org,
	"Theodore Ts'o" <tytso@....edu>, lkp@...ux.intel.com
Subject: Re: [PATCH] ext4: fix dirty pages writback regression.

On Tue, Sep 10, 2013 at 5:17 PM, Jan Kara <jack@...e.cz> wrote:
> On Tue 10-09-13 17:10:13, Yan, Zheng wrote:
>> 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@...el.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@...el.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?
>   Yes, that is true. Sorry, I didn't realize this originally. But what
> difference would then your patch make?
>

Continuous dirty pages that are "!buffer_delay(bh) && !buffer_unwritten(bh)"
can be merged during writeback. I think the change will reduce number of
bio for workload that re-writes existing data.

Regards
Yan, Zheng

>                                                                 Honza
>
>> > 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
>> >>
>>
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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