linux-ext4 - Re: Question on fallocate/ftruncate sequence

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Message-ID: <5df78e1d0908281142r683b902ube65288df858695d@mail.gmail.com>
Date:	Fri, 28 Aug 2009 11:42:59 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Frank Mayhar <fmayhar@...gle.com>
Cc:	Andreas Dilger <adilger@....com>,
	Eric Sandeen <sandeen@...hat.com>,
	Curt Wohlgemuth <curtw@...gle.com>,
	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: Question on fallocate/ftruncate sequence

Sorry for joining the conversation late. Frank and I had a discussion on this
problem this morning. We wonder whether we can just add the checking
on whether i_blocks is consistent with i_size during truncate. Here is the
patch I tried and it seems to have solved the problem. I.e., the space reserved
in fallocate(KEEP_SIZE) is now freed in the next truncate.

--- git-linux/fs/attr.c	2009-05-20 18:05:55.000000000 -0700
+++ linux-2.6.30.5/fs/attr.c	2009-08-27 14:34:48.000000000 -0700
@@ -68,7 +68,8 @@ int inode_setattr(struct inode * inode,
 	unsigned int ia_valid = attr->ia_valid;

 	if (ia_valid & ATTR_SIZE &&
-	    attr->ia_size != i_size_read(inode)) {
+	    (attr->ia_size != i_size_read(inode) ||
+            attr->ia_size >> 9 < inode->i_blocks - 1)) {
 		int error = vmtruncate(inode, attr->ia_size);
 		if (error)
 			return error;

One thing I am not sure is whether adding this check in inode_setattr may
cause any problem in other cases. I saw inode_setattr is called at many
places as well as during ftruncate. Any opinions on this proposed solution?

Jiaying

On Thu, Jul 23, 2009 at 3:46 PM, Frank Mayhar<fmayhar@...gle.com> wrote:
> On Thu, 2009-07-23 at 15:56 -0600, Andreas Dilger wrote:
>> On Jul 23, 2009  11:05 -0700, Frank Mayhar wrote:
>> > On Thu, 2009-07-23 at 12:00 -0500, Eric Sandeen wrote:
>> > > Sorry I skimmed to fast, skipped over the fsck part.  But:
>> > >
>> > > # touch /mnt/test/testfile
>> > > # /root/fallocate -n -l 16m /mnt/test/testfile
>> > > # ls -l /mnt/test/testfile
>> > > -rw-r--r-- 1 root root 0 Jul 23 12:13 /mnt/test/testfile
>> > > # du -h /mnt/test/testfile
>> > > 16M       /mnt/test/testfile
>> > >
>> > > there doesn't seem to be a problem in fsck w/ block past EOF, or am I
>> > > missing something else?
>> >
>> > I was taking Andreas' word for it but now that you mention it, I see the
>> > same thing.  Andreas, did you have a specific case in mind?
>>
>> Ted and I had discussed this in the past, maybe he fixed e2fsck to not
>> change the file size when there are blocks allocated beyond EOF.  Having
>> a flag wouldn't be a terrible idea, IMHO, so that e2fsck can make a
>> better decision on whether the size or the blocks count are more correct.
>> I'm not dead set on it.
>
> For the moment I'm going to table the e2fsck change and make the flag
> memory-only.  It'll be easy enough to change this if and when you guys
> come to an agreement about what is right.
>
> As for the flag itself, I'll pick a bit that doesn't conflict with
> anything else and leave reconciling the already-conflicting bits to you
> guys.
> --
> Frank Mayhar <fmayhar@...gle.com>
> Google, Inc.
>
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