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Date:	Tue, 13 Oct 2015 09:06:45 +0300
From:	"Leonid V. Fedorenchik" <Leonid.Fedorenchik@...agon-software.com>
To:	Theodore Ts'o <tytso@....edu>
CC:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	"Linux Kernel Developers List" <linux-kernel@...r.kernel.org>,
	<dave.hansen@...el.com>, <torvalds@...ux-foundation.org>,
	<akpm@...ux-foundation.org>
Subject: Re: [PATCH] ext4: use private version of page_zero_new_buffers()
 for data=journal mode

On Fri,  9 Oct 2015 00:01:09 -0400
Theodore Ts'o <tytso@....edu> wrote:

> If there is a error while copying data from userspace into the page
> cache during a write(2) system call, in data=journal mode, in
> ext4_journalled_write_end() were using page_zero_new_buffers() from
> fs/buffer.c.  Unfortunately, this sets the buffer dirty flag, which is
> no good if journalling is enabled.  This is a long-standing bug that
> goes back for years and years in ext3, but a combination of (a)
> data=journal not being very common, (b) in many case it only results
> in a warning message. and (c) only very rarely causes the kernel hang,
> means that we only really noticed this as a problem when commit
> 998ef75ddb caused this failure to happen frequently enough to cause
> generic/208 to fail when run in data=journal mode.
> 
> The fix is to have our own version of this function that doesn't call
> mark_dirty_buffer(), since we will end up calling
> ext4_handle_dirty_metadata() on the buffer head(s) in questions very
> shortly afterwards in ext4_journalled_write_end().
> 
> Thanks to Dave Hansen and Linus Torvalds for helping to identify the
> root cause of the problem.
> 
> Signed-off-by: Theodore Ts'o <tytso@....edu>
> ---
>  fs/ext4/inode.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index ae52e32..0a589bb 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1181,6 +1181,38 @@ errout:
>  	return ret ? ret : copied;
>  }
>  
> +/*
> + * This is a private version of page_zero_new_buffers() which doesn't
> + * set the buffer to be dirty, since in data=journalled mode we need
> + * to call ext4_handle_dirty_metadata() instad.
Small typo: s/instad/instead/

> + */
> +static void zero_new_buffers(struct page *page, unsigned from, unsigned to)
> +{
> +	unsigned int block_start = 0, block_end;
> +	struct buffer_head *head, *bh;
> +
> +	bh = head = page_buffers(page);
> +	do {
> +		block_end = block_start + bh->b_size;
> +		if (buffer_new(bh)) {
> +			if (block_end > from && block_start < to) {
> +				if (!PageUptodate(page)) {
> +					unsigned start, size;
> +
> +					start = max(from, block_start);
> +					size = min(to, block_end) - start;
> +
> +					zero_user(page, start, size);
> +					set_buffer_uptodate(bh);
> +				}
> +				clear_buffer_new(bh);
> +			}
> +		}
> +		block_start = block_end;
> +		bh = bh->b_this_page;
> +	} while (bh != head);
> +}
> +
>  static int ext4_journalled_write_end(struct file *file,
>  				     struct address_space *mapping,
>  				     loff_t pos, unsigned len, unsigned copied,
> @@ -1207,7 +1239,7 @@ static int ext4_journalled_write_end(struct file *file,
>  		if (copied < len) {
>  			if (!PageUptodate(page))
>  				copied = 0;
> -			page_zero_new_buffers(page, from+copied, to);
> +			zero_new_buffers(page, from+copied, to);
>  		}
>  
>  		ret = ext4_walk_page_buffers(handle, page_buffers(page), from,



-- 

Best regards,
Leonid Fedorenchik

Software Engineer
Paragon Software Group
Skype: leonid.fedorenchik
http://www.paragon-software.com

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