linux-ext4 - Re: [PATCH -v2] ext4: use truncate

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Date:	Tue, 24 May 2011 15:06:49 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH -v2] ext4: use truncate_setsize() unconditionally

On Tue, May 24, 2011 at 7:30 AM, Eric Sandeen <sandeen@...hat.com> wrote:
> On 5/23/11 2:19 PM, Theodore Ts'o wrote:
>> In commit c8d46e41 (ext4: Add flag to files with blocks intentionally
>> past EOF), if the EOFBLOCKS_FL flag is set, we call ext4_truncate()
>> before calling vmtruncate().  This caused any allocated but unwritten
>> blocks created by calling fallocate() with the FALLOC_FL_KEEP_SIZE
>> flag to be dropped.  This was done to make to make sure that
>> EOFBLOCKS_FL would not be cleared while still leaving blocks past
>> i_size allocated.  This was not necessary, since ext4_truncate()
>> guarantees that blocks past i_size will be dropped, even in the case
>> where truncate() has increased i_size before calling ext4_truncate().
>>
>> So fix this by removing the EOFBLOCKS_FL special case treatment in
>> ext4_setattr().  In addition, use truncate_setsize() followed by a
>> call to ext4_truncate() instead of using vmtruncate().  This is more
>> efficient since it skips the call to inode_newsize_ok(), which has
>> been checked already by inode_change_ok().  This is also in a win in
>> the case where EOFBLOCKS_FL is set since it avoids calling
>> ext4_truncate() twice.
>>
>> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
>> ---
>>  Jiayingz pointed out that in the case where we fallocate 12k, write 4k, and
>>  then truncate to 4k, we should discard the excess fallocate'd blocks.  So if
>>  attr->ia_size == inode.i_size, we can skip the truncate_setsize() call, but
>>  if the EOFBLOCKS_FL flag is set, we should still call ext4_truncate().
>
> are there xfstests which cover this explicitly?  It should be simple to write.
>
Vivek has written a xfstest to cover this and more fallocate/truncate cases:
http://old.nabble.com/-PATCH--xfstests%3A-test-fallocate%2C-write%2C-ftruncate-combinations.-to31666685.html#a31666685

Jiaying

> If filesystem behavior differs we can always make ext4-only tests.
>
> -Eric
>
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