Re: [RFC] [PATCH] Inverse locking order of page_lock and transaction start

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Thu, Mar 27, 2008 at 05:27:42PM +0100, Jan Kara wrote:
>   Hi,
> 
>   below is the first version of the patch that reverses locking order of
> page_lock and transaction start. I have tested it with fsx-linux, ltp DIO
> tests etc. and lockdep didn't complain so hopefully I got it mostly right
> but review is definitely needed. Especially I'd like to know what people
> think about the way I've implemented ext3_page_mkwrite() - ext4 has
> an incorrect code AFAICT because in ordered and journaled modes we should
> write block of zeros and properly journal it (and no, block_page_mkwrite()
> doesn't do it). We could implement ext3/4_page_mkwrite() in a similar way
> we currently implement writepage calls but calling write_begin + write_end
> does the job and should be only a tiny bit slower...
>   If nobody finds a serious flaw in the approach, I'll rediff the patch
> against ext4 (I'll also try to convert delayed-alloc path - from a quick
> look converting da_writepages path is going to be interesting).
>   I'm looking forward to your comments :)
> 
> 								Honza
> -- 
> Jan Kara <jack@suse.cz>
> SUSE Labs, CR
> 
> ---
> 
> Reverse locking order of page lock and transaction start in ext3  (i.e., page
> lock now comes after the transaction start). Needed changes are:
>   1) Simply swap the order in ext3_write_begin() and ext3_..._write_end()
>      (allows removal of ext3_generic_write_end())
>   2) Implement ext3_page_mkwrite() to fill holes.
>   3) Change ext3_writeback_writepage() not to start a transaction at all,
>      ext3_ordered_writepage() starts a transaction only after unlocking
>      the page in block_write_full_page() (to attach buffers to the transaction),
>      ext3_journaled_writepage() gets references to buffers in the page, unlocks
>      the page and then starts a transaction.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>
> 
> ---
>  fs/ext3/file.c          |   19 ++++-
>  fs/ext3/inode.c         |  236 +++++++++++++++++++++++++----------------------
>  include/linux/ext3_fs.h |    1 +
>  3 files changed, 145 insertions(+), 111 deletions(-)
> 

....

> +
> +static int ext3_bh_mapped(handle_t *handle, struct buffer_head *bh)
> +{
> +	return !buffer_mapped(bh);
> +}
> +
> +int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> +{
> +	struct file *file = vma->vm_file;
> +	struct inode *inode = file->f_path.dentry->d_inode;
> +	struct address_space *mapping = inode->i_mapping;
> +	unsigned long len;
> +	loff_t size;
> +	int ret = -EINVAL;
> +
> +	/*
> +	 * Get i_alloc_sem to stop truncates messing with the inode. We cannot
> +	 * get i_mutex because we are already holding mmap_sem. This makes
> +	 * it possible for write_begin and write_end to run concurrently
> +	 * on a single file (not on a single page because of page_lock).
> +	 * We seem to handle this just fine...
> +	 */
> +	down_read(&inode->i_alloc_sem);
> +	size = i_size_read(inode);
> +	if (page->mapping != mapping || size <= page_offset(page)
> +	    || !PageUptodate(page)) {
> +		/* page got truncated from under us? */
> +		goto out_unlock;
> +	}
> +	ret = 0;
> +	if (PageMappedToDisk(page))
> +		goto out_unlock;
> +
> +	if (page->index == size >> PAGE_CACHE_SHIFT)
> +		len = size & ~PAGE_CACHE_MASK;
> +	else
> +		len = PAGE_CACHE_SIZE;
> +
> +	if (page_has_buffers(page)) {
> +		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
> +				       ext3_bh_mapped))
> +			goto out_unlock;
> +	}
> +
> +	/* OK, we need to fill the hole... We simply write the page. */
> +	printk(KERN_INFO "Writing page %lu of ino %lu\n", page->index, inode->i_ino);
> +	ret = mapping->a_ops->write_begin(file, mapping, page_offset(page),
> +		len, AOP_FLAG_UNINTERRUPTIBLE, &page, NULL);
> +	if (ret < 0)
> +		goto out_unlock;
> +	ret = mapping->a_ops->write_end(file, mapping, page_offset(page), len,
> +		len, page, NULL);
> +	if (ret < 0)
> +		goto out_unlock;
> +	ret = 0;
> +out_unlock:
> +	up_read(&inode->i_alloc_sem);
> +	return ret;
> +}
> diff --git a/include/linux/ext3_fs.h b/include/linux/ext3_fs.h
> index 36c5403..715c35e 100644
> --- a/include/linux/ext3_fs.h
> +++ b/include/linux/ext3_fs.h
> @@ -836,6 +836,7 @@ extern void ext3_truncate (struct inode *);
>  extern void ext3_set_inode_flags(struct inode *);
>  extern void ext3_get_inode_flags(struct ext3_inode_info *);
>  extern void ext3_set_aops(struct inode *inode);
> +extern int ext3_page_mkwrite(struct vm_area_struct *vma, struct page *page);
> 
>  /* ioctl.c */
>  extern int ext3_ioctl (struct inode *, struct file *, unsigned int,

The comments on block_page_mkwrite says taking a lock on page protect it
against truncate. Why do we need to take i_alloc_sem ? Is it because
after changing the locking order we can't any more take the page lock
here because we need to take it after the transaction is started ?

My patch to use page_mkwrite on ext3 resulted in this discussion.
http://article.gmane.org/gmane.comp.file-systems.ext4/5731

Does the above means that it will call page_mkwrite with page lock held.
That would imply that we can't start transaction inside page_mkwrite

Why do you think that current Ext4 code page_mkwrite is wrong ?
We just need to reserve space for the page we are dirtying right.

I have tried a similar change and later dropped it because we didn't
had much anything to journal 
http://article.gmane.org/gmane.comp.file-systems.ext4/5201
This had the inode_lock taken which lockdep complained about.

ext4_get_blocks create a journal handle for all meta update if we don't
have one.

-aneesh