[Ocfs2-devel] [PATCH] ocfs2: make __ocfs2_page_mkwrite handle file end properly.

[Joel Becker]

On Fri, Jul 16, 2010 at 10:21:00PM +0800, Tao Ma wrote:
> __ocfs2_page_mkwrite now is broken in handling file end.
> 1. the last page should be the page contains i_size - 1.
> 2. the len in the last page is also calculated wrong.
> So change them accordingly.

	How did you determine these broken?  They look correct to me,
and they passed the mmap testing I ran against the tail zeroing fixes.
Comments inline.

> diff --git a/fs/ocfs2/mmap.c b/fs/ocfs2/mmap.c
> index af2b8fe..4c18f4a 100644
> --- a/fs/ocfs2/mmap.c
> +++ b/fs/ocfs2/mmap.c
> @@ -74,9 +74,11 @@ static int __ocfs2_page_mkwrite(struct inode *inode, struct buffer_head *di_bh,
>  	/*
>  	 * Another node might have truncated while we were waiting on
>  	 * cluster locks.
> +	 * We don't check size == 0 before the shift. This is borrowed
> +	 * from do_generic_file_read.
>  	 */
> -	last_index = size >> PAGE_CACHE_SHIFT;
> -	if (page->index > last_index) {

	This original check seems correct.  If size is on a page
boundary, sure the last_index is past the last page of the file.
That's OK, we're checking that page->index isn't greater than that.

> @@ -107,7 +109,7 @@ static int __ocfs2_page_mkwrite(struct inode *inode, struct buffer_head *di_bh,
>  	 * because the "write" would invalidate their data.
>  	 */
>  	if (page->index == last_index)
> -		len = size & ~PAGE_CACHE_MASK;
> +		len = ((size - 1) & ~PAGE_CACHE_MASK) + 1;

	And the len calculation is only broken because you changed what
last_index meant.  Originally, if the page equals the last_index, size
cannot be page aligned, and you are guaranteed a proper len.
	You know, if you don't like the way last_index reads, we can
always steal the ext4 comparison:

5982         if (page->mapping != mapping || size <= page_offset(page)
5983             || !PageUptodate(page)) {
5984                 /* page got truncated from under us? */
5985                 goto out_unlock;
5986         }

<snip>

5990 
5991         if (page->index == size >> PAGE_CACHE_SHIFT)
5992                 len = size & ~PAGE_CACHE_MASK;
5993         else
5994                 len = PAGE_CACHE_SIZE;

	This is a direct compare on the page_offset, which doesn't
confuse anyone about index vs i_size.  Later, they directly check "is
this page the last in the file" before computing len.

Joel

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Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127