On Fri, 27 Jul 2012, Lukas Czerner 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!
I'm glad you have found that i_mutex really is needed here: it had
worried me that it was not taken, but I could only raise a concern,
didn't really know one way or the other.
>
> Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> ---
> 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 91341ec..2d6a216 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -4755,9 +4755,11 @@ 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;
Note that this is wrong, but there is no reason why it should be you
to fix it in this patchset. Blocks may have been fallocated beyond
i_size, and they should be removed when a hole is punched there.
It's on the ext4 TODO list to be fixed, so don't worry about it:
unless your changes happen to make it trivial to fix at the same time.
>
> /*
> * If the hole extends beyond i_size, set the hole
> @@ -4775,18 +4777,6 @@ 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;
> - }
> -
It's not clear to me why that's now safe to remove: a little more comment
in the commit would be good; but so long as it's clear to ext4 developers,
don't try to make it clear to me - that would take far too long!
Hugh
> /* Now release the pages */
> if (last_page_offset > first_page_offset) {
> truncate_pagecache_range(inode, first_page_offset,
> @@ -4798,12 +4788,14 @@ 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
> @@ -4893,6 +4885,8 @@ 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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