Re: [PATCH 1 of 2] Implement generic block_page_mkwrite() functionality

[Hugh Dickins <hugh@veritas.com>]

On Wed, 7 Feb 2007, David Chinner wrote:

> On Christoph's suggestion, take the guts of the proposed
> xfs_vm_page_mkwrite function and implement it as a generic
> core function as it used no specific XFS code at all.
> 
> This allows any filesystem to easily hook the ->page_mkwrite()
> VM callout to allow them to set up pages dirtied by mmap
> writes correctly for later writeout.
> 
> Signed-Off-By: Dave Chinner <dgc@sgi.com>

I'm worried about concurrent truncation.  Isn't it the case that
i_mutex is held when prepare_write and commit_write are normally
called?  But not here when page_mkwrite is called.

Hugh

> 
> ---
>  fs/buffer.c                 |   30 ++++++++++++++++++++++++++++++
>  include/linux/buffer_head.h |    2 ++
>  2 files changed, 32 insertions(+)
> 
> Index: 2.6.x-xfs-new/fs/buffer.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/buffer.c	2007-02-07 23:00:05.000000000 +1100
> +++ 2.6.x-xfs-new/fs/buffer.c	2007-02-07 23:09:47.642356116 +1100
> @@ -2194,6 +2194,36 @@ int generic_commit_write(struct file *fi
>  	return 0;
>  }
>  
> +/*
> + * block_page_mkwrite() is not allowed to change the file size as
> + * it gets called from a page fault handler when a page is first
> + * dirtied. Hence we must be careful to check for EOF conditions
> + * here. We set the page up correctly for a written page which means
> + * we get ENOSPC checking when writing into holes and correct
> + * delalloc and unwritten extent mapping on filesystems that support
> + * these features.
> + */
> +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, get_block);
> +	if (!ret)
> +		ret = block_commit_write(page, 0, end);
> +	unlock_page(page);
> +
> +	return ret;
> +}
>  
>  /*
>   * nobh_prepare_write()'s prereads are special: the buffer_heads are freed
> Index: 2.6.x-xfs-new/include/linux/buffer_head.h
> ===================================================================
> --- 2.6.x-xfs-new.orig/include/linux/buffer_head.h	2007-02-07 23:00:02.000000000 +1100
> +++ 2.6.x-xfs-new/include/linux/buffer_head.h	2007-02-07 23:12:33.156749344 +1100
> @@ -206,6 +206,8 @@ int cont_prepare_write(struct page*, uns
>  int generic_cont_expand(struct inode *inode, loff_t size);
>  int generic_cont_expand_simple(struct inode *inode, loff_t size);
>  int block_commit_write(struct page *page, unsigned from, unsigned to);
> +int block_page_mkwrite(struct vma_area_struct *vma, struct page *page,
> +				get_block_t get_block);
>  void block_sync_page(struct page *);
>  sector_t generic_block_bmap(struct address_space *, sector_t, get_block_t *);
>  int generic_commit_write(struct file *, struct page *, unsigned, unsigned);

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