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

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Message-ID: <BANLkTi=0NSws0GV4hP5KPPqvp4bxOSEbaQ@mail.gmail.com>
Date:	Mon, 23 May 2011 11:30:29 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] ext4: use vmtruncate() instead of ext4_truncate() in ext4_setattr()

Hi Ted,

On Sun, May 22, 2011 at 4:56 PM, Ted Ts'o <tytso@....edu> wrote:
> Here's a cleaner way of fixing the problem.  vmtruncate() is now
> deprecated, so I'm using truncate_setsize() instead; in addition, I
> avoid calling ext4_truncate() twice in the case where EOFBLOCKS_FL.
> Since ext4_truncate() is a very heavyweight function, which requires
> manipulating the orphan list and taking i_data_sem, avoiding a double
> call of ext4_truncate() is a big win.
>
>                                                - Ted
>
> commit bb5eabd2de1e9dfdeeac822174fba17f2d2a7f2b
> Author: Theodore Ts'o <tytso@....edu>
> Date:   Sun May 22 19:45:01 2011 -0400
>
>    ext4: use truncate_setsize() unconditionally
>
>    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>
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index df3fb20..4ca8411 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -5363,8 +5363,7 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
>
>        if (S_ISREG(inode->i_mode) &&
>            attr->ia_valid & ATTR_SIZE &&
> -           (attr->ia_size < inode->i_size ||
> -            (ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS)))) {
> +           (attr->ia_size < inode->i_size)) {
>                handle_t *handle;
>
>                handle = ext4_journal_start(inode, 3);
> @@ -5398,14 +5397,13 @@ int ext4_setattr(struct dentry *dentry, struct iattr *attr)
>                                goto err_out;
>                        }
>                }
> -               /* ext4_truncate will clear the flag */
> -               if ((ext4_test_inode_flag(inode, EXT4_INODE_EOFBLOCKS)))
> -                       ext4_truncate(inode);
>        }
>
>        if ((attr->ia_valid & ATTR_SIZE) &&
> -           attr->ia_size != i_size_read(inode))
> -               rc = vmtruncate(inode, attr->ia_size);
> +           attr->ia_size != i_size_read(inode)) {
> +               truncate_setsize(inode, attr->ia_size);
> +               ext4_truncate(inode);
> +       }
I think your patch doesn't cover the case when we fallocate with
KEEP_SIZE a 12k file, write 4k, and then truncate the file to 4k.
In this case, attr->ia_size is equal to i_size but we still want to
free any allocated but unwritten blocks during truncate.

Jiaying
>
>        if (!rc) {
>                setattr_copy(inode, attr);
>
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