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

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Date:	Fri, 28 Aug 2009 14:44:23 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Andreas Dilger <adilger@....com>
Cc:	Frank Mayhar <fmayhar@...gle.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

On Fri, Aug 28, 2009 at 12:40 PM, Andreas Dilger<adilger@....com> wrote:
> On Aug 28, 2009  11:42 -0700, Jiaying Zhang wrote:
>> 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;
>
> This isn't really correct, however, because i_blocks also contains
> non-data blocks (indirect/index, EA, etc) blocks, so even with small
> files with ACLs i_blocks may always be larger than ia_size >> 9, and
> for ext2/3 at least this will ALWAYS be true for files > 48kB in size.

I see. I guess we need to use a special flag then. Or is there any
other suggestions? I also have another question related to this
problem. Why those fallocated blocks are not marked as preallocated
blocks that will then be automatically freed in ext4_release_file?

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.
>> >
>> > --
>> > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
>> > the body of a message to majordomo@...r.kernel.org
>> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> >
>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>
>
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