linux-ext4 - Re: [PATCH v2] ext4: avoid redundant cache finding if flags not cantian PUNCH

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Date:	Mon, 27 Jun 2011 17:44:58 -0700
From:	Eric Gouriou <egouriou@...gle.com>
To:	Robin Dong <hao.bigrat@...il.com>
Cc:	djwong@...ibm.com, linux-ext4@...r.kernel.org,
	Robin Dong <sanbai@...bao.com>
Subject: Re: [PATCH v2] ext4: avoid redundant cache finding if flags not
 cantian PUNCH_OUT

On Mon, Jun 27, 2011 at 12:59, Darrick J. Wong <djwong@...ibm.com> wrote:
> On Mon, Jun 27, 2011 at 06:45:13PM +0800, Robin Dong wrote:
>> Subject: [PATCH v2] ext4: avoid redundant cache finding if flags not cantian
>
> I suspect you mean "contain" as opposed to "cantian".

The patch summary line could also use some shortening.

>> In old code, ext4 will find extent in cache first even flags does
>> not contian PUNCH_OUT, that may waste some time.
>
> Same here ("contian").

How about something along the lines of:
---------
ext4: avoid wasted extent cache lookup if !PUNCH_OUT_EXT

This patch avoids an extraneous lookup of the extent cache
in ext4_ext_map_blocks() when the flag
EXT4_GET_BLOCKS_PUNCH_OUT_EXT is absent.

The existing logic was performing the lookup but not making
use of the result. The patch simply reverses the order of evaluation
in the condition.

Since ext4_ext_in_cache() does not initialize newex on misses, bypassing
its invocation does not introduce any new issue in this regard.
---------

 Regards - Eric

>
> --D
>>
>> Signed-off-by: Robin Dong <sanbai@...bao.com>
>> Reviewed-by: Lukas Czerner <lczerner@...hat.com>
>> ---
>>  fs/ext4/extents.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
>> index eb63c7b..fed2633 100644
>> --- a/fs/ext4/extents.c
>> +++ b/fs/ext4/extents.c
>> @@ -3358,8 +3358,8 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
>>       trace_ext4_ext_map_blocks_enter(inode, map->m_lblk, map->m_len, flags);
>>
>>       /* check in cache */
>> -     if (ext4_ext_in_cache(inode, map->m_lblk, &newex) &&
>> -             ((flags & EXT4_GET_BLOCKS_PUNCH_OUT_EXT) == 0)) {
>> +     if (!(flags & EXT4_GET_BLOCKS_PUNCH_OUT_EXT) &&
>> +             ext4_ext_in_cache(inode, map->m_lblk, &newex)) {
>>               if (!newex.ee_start_lo && !newex.ee_start_hi) {
>>                       if ((flags & EXT4_GET_BLOCKS_CREATE) == 0) {
>>                               /*
>> --
>> 1.7.1
>>
>> --
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