Re: Question on fallocate/ftruncate sequence

[Andreas Dilger <adilger@sun.com>]

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.

> On Thu, Jul 23, 2009 at 3:46 PM, Frank Mayhar<fmayhar@google.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@google.com>
> > Google, Inc.
> >
> > --
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> >

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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