On Wed, Feb 07, 2007 at 09:53:25AM +1100, David Chinner wrote:
> Folks,
>
> I'm not sure of the exact locking rules and constraints for
> ->page_mkwrite(), so I thought I better fish around for comments.
>
> With XFS, we need to hook pages being dirtied by mmap writes so that
> we can attach buffers of the correct state tothe pages. This means
> that when we write them back, the correct thing happens.
>
> For example, if you mmap an unwritten extent (preallocated),
> currently your data will get written to disk but the extent will not
> get converted to a written extent. IOWs, you lose the data because
> when you read it back it will seen as unwritten and treated as a
> hole.
>
> AFAICT, it is safe to lock the page during ->page_mkwrite and that
> it is safe to issue I/O (e.g. metadata reads) to determine the
> current state of the file. I am also assuming that, at this point,
> we are not allowed to change the file size and so we have to be
> careful in ->page_mkwrite we don't do that. What else have I missed
> here?
>
> IOWs, I've basically treated ->page_mkwrite() as wrapper for
> block_prepare_write/block_commit_write because they do all the
> buffer mapping and state manipulation I think is necessary. Is it
> safe to call these functions, or are there some other constraints we
> have to work under here?
>
> Patch below. Comments?
>
> Cheers,
>
> Dave.
> --
> Dave Chinner
> Principal Engineer
> SGI Australian Software Group
>
>
> ---
> fs/xfs/linux-2.6/xfs_file.c | 34 ++++++++++++++++++++++++++++++++++
> 1 file changed, 34 insertions(+)
>
> Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_file.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_file.c 2007-01-16
> 10:54:15.000000000 +1100
> +++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_file.c 2007-02-07 09:49:00.508017483
> +1100
> @@ -446,6 +446,38 @@ xfs_file_open_exec(
> }
> #endif /* HAVE_FOP_OPEN_EXEC */
>
> +/*
> + * mmap()d file has taken write protection fault and is being made
> + * writable. We can set the page state up correctly for a writable
> + * page, which means we can do correct delalloc accounting (ENOSPC
> + * checking!) and unwritten extent mapping.
> + */
> +STATIC int
> +xfs_vm_page_mkwrite(
> + struct vm_area_struct *vma,
> + struct page *page)
> +{
> + struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
> + unsigned long end;
> + int ret = 0;
> +
> + end = page->index + 1;
> + end <<= PAGE_CACHE_SHIFT;
> + if (end > i_size_read(inode))
> + end = i_size_read(inode) & ~PAGE_CACHE_MASK;
> + else
> + end = PAGE_CACHE_SIZE;
> +
> + lock_page(page);
> + ret = block_prepare_write(page, 0, end, xfs_get_blocks);
> + if (!ret)
> + ret = block_commit_write(page, 0, end);
> + unlock_page(page);
> +
> + return ret;
> +}
This looks to me. But given that this is generic code except for the
get_block callback, shouldn't we put the guts into buffer.c and wire
all filesystems up to use it? e.g.
int block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
get_block_t get_block)
{
struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
unsigned long end;
int ret = 0;
if ((page->index + 1) << PAGE_CACHE_SHIFT > i_size_read(inode))
end = i_size_read(inode) & ~PAGE_CACHE_MASK;
else
end = PAGE_CACHE_SIZE;
lock_page(page);
ret = block_prepare_write(page, 0, end, block);
if (!ret)
ret = block_commit_write(page, 0, end);
unlock_page(page);
return ret;
}
and then in xfs and similar in other filesystems:
STATIC int
xfs_vm_page_mkwrite(
struct vm_area_struct *vma,
struct page *page)
{
return block_page_mkwrite(vma, page, xfs_get_blocks);
}
BTW, why is xfs_get_blocks not called xfs_get_block?
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