linux-ext4 - Re: [PATCH] ext4: restart ext4_ext_remove_space() after transaction restart V2

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Message-ID: <20100526134603.GA3494@quack.suse.cz>
Date:	Wed, 26 May 2010 15:46:03 +0200
From:	Jan Kara <jack@...e.cz>
To:	tytso@....edu
Cc:	Dmitry Monakhov <dmonakhov@...nvz.org>, linux-ext4@...r.kernel.org,
	jack@...e.cz, aneesh.kumar@...ux.vnet.ibm.com
Subject: Re: [PATCH] ext4: restart ext4_ext_remove_space() after
 transaction restart V2

On Wed 26-05-10 09:23:52, tytso@....edu wrote:
> One more thing.  Why do you need EXT4_STATE_EXT_TRUNC?
> 
> The only place which tests it in any kind of real way is
> ext4_ext_truncate_extend_restart(), and it is only called by one
> function, ext4_ext_rm_leaf(), and *it* is only called in one place,
> inside ext4_ext_remove_space(), and *it* surronds the call with
> ext4_set_inode_state(inode, EXT4_STATE_EXT_TRUNC) and
> ext4_clear_inode_state(inode, EXT4_STATE_EXT_TRUNC).
> 
> And while a truncate is happening, no other block allocation can
> happen, so the test in ext4_ext_map_blocks() doesn't seem to do much.
  This is false. As soon as we drop i_data_sem, allocation *can* happen
from writeback path. Because truncate has already invalidated all the pages
past new_size, it must be for some page before new_size but still it could
modify an extent tree node we passed through when looking up our extent...

> (It only clears STATE_EXT_TRUNC if it is set and if the flags
> EXT4_GET_BLOCKS_CREATE is set.  I'm not sure what the point of that
> is, either.)
  I think the idea Dmitry tries to implement is:
When allocation like I describe above happens while we droppped i_data_sem,
restart the whole truncation.

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