linux-ext4 - Re: [PATCH 1/2] block: add BH

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Date:	Mon, 14 May 2012 10:24:39 +0530
From:	Saugata Das <saugata.das@...aro.org>
To:	Namhyung Kim <namhyung.kim@....com>
Cc:	Saugata Das <saugata.das@...ricsson.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	tytso@....edu, adilger@...mcloud.com, hch@...radead.org,
	arnd.bergmann@...aro.org, venkat@...aro.org
Subject: Re: [PATCH 1/2] block: add BH_Meta flag

On 14 May 2012 07:12, Namhyung Kim <namhyung.kim@....com> wrote:
> Hi,
>
> On Fri, 11 May 2012 20:32:56 +0530, Saugata Das wrote:
>> From: Saugata Das <saugata.das@...aro.org>
>>
>> Today, storage devices like eMMC has special features like data tagging
>> (introduced in MMC-4.5 version) in order to improve performance of some
>> specific writes. On MMC stack, data tagging is used for all writes which
>> has REQ_META flag set. This patch adds the capability to add REQ_META flag
>> during meta data write.
>>
>
> AFAIK, the REQ_META is only for marking a bio/req to be recognized when
> using blktrace or something. You can use REQ_PRIO for the purpose but it
> applies only if your ioscheduler is CFQ.
>
> But I'm not aware how the MMC stack works, so I might be missing something.
>

Today on ext4, REQ_META or REQ_PRIO are only used during read
operation. For meta-data writes, no special flag (REQ_META or
REQ_PRIO) is set.

On eMMC, we depend on REQ_META flag to implement the "reliable write"
and "data tag" feature, which are linked with additional reliability
and performance for file system meta-data writes.


> Thanks,
> Namhyung
>
>
>> Signed-off-by: Saugata Das <saugata.das@...aro.org>
>> ---
>>  fs/buffer.c                 |   10 ++++++++--
>>  include/linux/buffer_head.h |    2 ++
>>  2 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/buffer.c b/fs/buffer.c
>> index 36d6665..688b38b 100644
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -1685,7 +1685,10 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
>>       do {
>>               struct buffer_head *next = bh->b_this_page;
>>               if (buffer_async_write(bh)) {
>> -                     submit_bh(write_op, bh);
>> +                     if (buffer_meta(bh))
>> +                             submit_bh(write_op | REQ_META, bh);
>> +                     else
>> +                             submit_bh(write_op, bh);
>>                       nr_underway++;
>>               }
>>               bh = next;
>> @@ -1739,7 +1742,10 @@ recover:
>>               struct buffer_head *next = bh->b_this_page;
>>               if (buffer_async_write(bh)) {
>>                       clear_buffer_dirty(bh);
>> -                     submit_bh(write_op, bh);
>> +                     if (buffer_meta(bh))
>> +                             submit_bh(write_op | REQ_META, bh);
>> +                     else
>> +                             submit_bh(write_op, bh);
>>                       nr_underway++;
>>               }
>>               bh = next;
>> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
>> index ef26043..0776564 100644
>> --- a/include/linux/buffer_head.h
>> +++ b/include/linux/buffer_head.h
>> @@ -34,6 +34,7 @@ enum bh_state_bits {
>>       BH_Write_EIO,   /* I/O error on write */
>>       BH_Unwritten,   /* Buffer is allocated on disk but not written */
>>       BH_Quiet,       /* Buffer Error Prinks to be quiet */
>> +     BH_Meta,        /* Is meta */
>>
>>       BH_PrivateStart,/* not a state bit, but the first bit available
>>                        * for private allocation by other entities
>> @@ -126,6 +127,7 @@ BUFFER_FNS(Delay, delay)
>>  BUFFER_FNS(Boundary, boundary)
>>  BUFFER_FNS(Write_EIO, write_io_error)
>>  BUFFER_FNS(Unwritten, unwritten)
>> +BUFFER_FNS(Meta, meta)
>>
>>  #define bh_offset(bh)                ((unsigned long)(bh)->b_data & ~PAGE_MASK)
>>  #define touch_buffer(bh)     mark_page_accessed(bh->b_page)
--
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