linux-ext4 - Re: [PATCH v2] REQ_META set during meta data write

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Date:	Thu, 22 Mar 2012 16:38:07 +0530
From:	Saugata Das <saugata.das@...aro.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Saugata Das <saugata.das@...ricsson.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	arnd.bergmann@...aro.org, tytso@....edu, adilger@...mcloud.com,
	venkat@...aro.org
Subject: Re: [PATCH v2] REQ_META set during meta data write

Hi Christoph

On 4 March 2012 13:32, Saugata Das <saugata.das@...aro.org> wrote:
> On 3 March 2012 15:04, Christoph Hellwig <hch@...radead.org> wrote:
>> On Fri, Mar 02, 2012 at 06:15:09PM +0530, Saugata Das wrote:
>>> --- a/fs/buffer.c
>>> +++ b/fs/buffer.c
>>> @@ -2926,6 +2926,9 @@ int submit_bh(int rw, struct buffer_head * bh)
>>>       if (test_set_buffer_req(bh) && (rw & WRITE))
>>>               clear_buffer_write_io_error(bh);
>>>
>>> +     if (buffer_meta(bh))
>>> +             rw |= REQ_META;
>>> +
>>
>> submit_bh has a perfectly valid rw argument where you can pass REQ_META.
>> Please use that instead of abusing a buffer state bit for an I/O
>> submission flag.
>>
>
> Typically, during meta-data writes, the buffer is only marked dirty
> (__ext4_handle_dirty_metadata). The submit_bh of the dirty meta-data
> buffer happens later from writeback thread which in general flushes
> out all the buffers. From this generic handling of dirty buffers from
> writeback thread via generic writepage functions till submit_bh, it is
> not possible to identify whether the buffer was corresponding to any
> meta-data without having an extra information in the buffer itself.

Is the above explanation OK for you ? Or, do you some alternative
suggestion for me to explore ?

Based on your feedback, I shall resubmit the patch on top of 3.4.


regards
saugata
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