linux-ext4 - Re: [PATCH] ext4: use vmtruncate() instead of ext4_truncate() in ext4

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Date:	Wed, 18 May 2011 13:29:23 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	tytso@....edu, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: use vmtruncate() instead of ext4_truncate() in ext4_setattr()

On Tue, May 17, 2011 at 8:19 PM, Eric Sandeen <sandeen@...hat.com> wrote:
>
> On 5/17/11 5:59 PM, Jiaying Zhang wrote:
> > There is a bug in commit c8d46e41 "ext4: Add flag to files with blocks
> > intentionally past EOF" that if we fallocate a file with FALLOC_FL_KEEP_SIZE
> > flag and then ftruncate the file to a size larger than the file's i_size,
> > any allocated but unwritten blocks will be freed but the file size is set
> > to the size that ftruncate specifies.
> >
> > Here is a simple test to reproduce the problem:
> >   1. fallocate a 12k size file with KEEP_SIZE flag
> >   2. write the first 4k
> >   3. ftruncate the file to 8k
> > Then 'ls -l' shows that the i_size of the file becomes 8k but debugfs
> > shows the file has only the first written block left.
>
> To be honest I'm not 100% certain what the fiesystem -should- do in this case.
>
> If I go through that same sequence on xfs, I get 4k written / 8k unwritten:
>
> # xfs_bmap -vp testfile
> testfile:
>  EXT: FILE-OFFSET      BLOCK-RANGE            AG AG-OFFSET              TOTAL FLAGS
>   0: [0..7]:          2648750760..2648750767  3 (356066400..356066407)     8 00000
>   1: [8..23]:         2648750768..2648750783  3 (356066408..356066423)    16 10000
>
> size 8k:
> # ls -l testfile
> -rw-r--r-- 1 root root 8192 May 17 22:33 testfile
>
> and diskspace used 12k:
> # du -hc testfile
> 12K     testfile
> 12K     total
>
> I think this is a different result from ext4, either with or without your patch.
>
> On ext4 I get size 8k, but only the first 4k mapped, as you say.
I agree that truncating to a size larger than i_size is un-specified by
POSIX. However, I think the problem with the current behavior is that
we have an inconsistency between file's i_size and its extent tree.
Now we have 8k i_size but the file has only 4k space allocated. That
can confuse applications.

Jiaying
>
> I don't recall when truncate is supposed to free fallocated blocks, and from what point?
>
> -Eric
>
> > Below is the proposed patch to fix the bug:
> >
> > ext4: use vmtruncate() instead of ext4_truncate() in ext4_setattr().
> >
> > Change ext4_setattr() to use vmtruncate(inode, attr->ia_size) instead
> > of ext4_truncate(inode) when it needs to truncate an inode so that
> > if the inode has EXT4_EOFBLOCKS_FL flag set and we are trying to truncate
> > to a size larger than the inode's i_size, we will only truncate the blocks
> > beyond the specified truncate size instead of all of blocks beyond i_size.
> >
> > Signed-off-by: Jiaying Zhang <jiayingz@...gle.com>
> >
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 3424e82..3bfad57 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -5347,8 +5347,11 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
> >                       }
> >               }
> >               /* ext4_truncate will clear the flag */
> > -             if ((ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS)))
> > -                     ext4_truncate(inode);
> > +             if ((ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS))) {
> > +                     rc = vmtruncate(inode, attr->ia_size);
> > +                     if (rc)
> > +                             goto err_out;
> > +             }
> >       }
> >
> >       if ((attr->ia_valid & ATTR_SIZE) &&
> > --
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