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

Hugh Dickins
2007-02-07T13:16:34Z

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);