linux-ext4 - Re: [PATCH RFC] Insure direct IO writes do not use the page cache

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Date:	Thu, 30 Jul 2009 13:39:12 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	Theodore Tso <tytso@....edu>, Curt Wohlgemuth <curtw@...gle.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH RFC] Insure direct IO writes do not use the page cache

Jan Kara wrote:
>> On Tue, Jul 28, 2009 at 05:28:05PM -0700, Curt Wohlgemuth wrote:

...

>> 2) We can modify the ext4_ext_convert_to_initialized() to be more
>> aggressive about initializing data blocks if we know we are doing DIO,
>> since zero'ing an aligned 16 to 32 blocks and then waiting for the
>> journal commit once is cheaper than converting the extent one block at
>> a time and waiting for the journal commit after each block write.
>   Definitely. I'm not following the discussion too much in detail but
> what seems to me is the following could work:
>   The direct IO path would first send all the data to disk to the
> desired location (get_block wouldn't do any conversion, just map blocks).
> When this is done, we convert all the touched extents to initialized ones
> from ext4_direct_IO, update i_size if needed, and wait for transaction
> commit.
> 
> 								Honza

This is all about right, but it's tricky, because right now, get_block
is called in the direct IO path from get_more_blocks(), and it's called
with create == 0 unless OWN_LOCKING is specified.  If we do get_block w/
create == 0 and find prealloc'd blocks, then we're given back unmapped
buffer heads. This looks like a hole, and so DIO falls back to buffered.

Right now the only way to get create == 1 sent to get_blocks via
directio is to do OWN_LOCKING, which implies... we have to do our own
locking, and it'll take some time to get it right I think.

-Eric
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