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Date:	Mon, 10 Sep 2012 17:30:53 +0530
From:	Ashish Sangwan <ashishsangwan2@...il.com>
To:	Lukas Czerner <lczerner@...hat.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	tytso@....edu, hughd@...gle.com, linux-mm@...ck.org
Subject: Re: [PATCH 07/15 v2] ext4: Take i_mutex before punching hole

On Sat, Sep 1, 2012 at 3:51 AM, Lukas Czerner <lczerner@...hat.com> wrote:
> Currently the allocation might happen in the punched range after the
> truncation and before the releasing the space of the range. This would
> lead to blocks being unallocated under the mapped buffer heads resulting
> in nasty bugs.
>
> With this commit we take i_mutex before going to do anything in the
> ext4_ext_punch_hole() preventing any write to happen while the hole
> punching is in progress. This will also allow us to ditch the writeout
> of dirty pages withing the range.
>
> This commit was based on code provided by Zheng Liu, thanks!
>
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> ---
>  fs/ext4/extents.c |   26 ++++++++++----------------
>  1 files changed, 10 insertions(+), 16 deletions(-)
>
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index aabbb3f..f920383 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4769,9 +4769,11 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>         loff_t first_page_offset, last_page_offset;
>         int credits, err = 0;
>
> +       mutex_lock(&inode->i_mutex);
> +
>         /* No need to punch hole beyond i_size */
>         if (offset >= inode->i_size)
> -               return 0;
> +               goto out1;
>
>         /*
>          * If the hole extends beyond i_size, set the hole
> @@ -4789,18 +4791,6 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>         first_page_offset = first_page << PAGE_CACHE_SHIFT;
>         last_page_offset = last_page << PAGE_CACHE_SHIFT;
>
> -       /*
> -        * Write out all dirty pages to avoid race conditions
> -        * Then release them.
> -        */
> -       if (mapping->nrpages && mapping_tagged(mapping, PAGECACHE_TAG_DIRTY)) {
> -               err = filemap_write_and_wait_range(mapping,
> -                       offset, offset + length - 1);
> -
> -               if (err)
> -                       return err;
> -       }
> -

Removing above code will cause a problem in case the file has all its
data in memory and nothing has been committed on disk. If punch hole
is issued for such a file, as there are no extents present, EIO would
be returned from ext4_ext_rm_leaf. So, even though blocks would be
removed from memory, the end result will be error EIO.

>         /* Now release the pages */
>         if (last_page_offset > first_page_offset) {
>                 truncate_pagecache_range(inode, first_page_offset,

To avoid this, you can add a check after the call to truncate_pagecache_range.
if(!inode->i_blocks)
  return 0;

> @@ -4812,12 +4802,14 @@ int ext4_ext_punch_hole(struct file *file, loff_t offset, loff_t length)
>
>         credits = ext4_writepage_trans_blocks(inode);
>         handle = ext4_journal_start(inode, credits);
> -       if (IS_ERR(handle))
> -               return PTR_ERR(handle);
> +       if (IS_ERR(handle)) {
> +               err = PTR_ERR(handle);
> +               goto out1;
> +       }
>
>         err = ext4_orphan_add(handle, inode);
>         if (err)
> -               goto out;
> +               goto out1;
>
>         /*
>          * Now we need to zero out the non-page-aligned data in the
> @@ -4907,6 +4899,8 @@ out:
>         inode->i_mtime = inode->i_ctime = ext4_current_time(inode);
>         ext4_mark_inode_dirty(handle, inode);
>         ext4_journal_stop(handle);
> +out1:
> +       mutex_unlock(&inode->i_mutex);
>         return err;
>  }
>  int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
> --
> 1.7.7.6
>
> --
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