Re: [PATCH 10/22] CacheFiles: Add a hook to write a single page of data to an inode

[Trond Myklebust <Trond.Myklebust@netapp.com>]

On Fri, 2007-09-21 at 15:47 +0100, David Howells wrote:
> Add an address space operation to write one single page of data to an inode at
> a page-aligned location (thus permitting the implementation to be highly
> optimised).
> 
> This is used by CacheFiles to store the contents of netfs pages into their
> backing file pages.
> 
> Supply a generic implementation for this that uses the prepare_write() and
> commit_write() address_space operations to bound a copy directly into the page
> cache.
> 
> Hook the Ext2 and Ext3 operations to the generic implementation.

So why do you need a new address space operation? AFAICS the generic
implementation will work for pretty much everyone who supports the
existing prepare_write()/commit_write().
Furthermore, you don't appear to supply any alternative "optimised"
implementations...

Trond


> Signed-Off-By: David Howells <dhowells@redhat.com>
> ---
> 
>  fs/ext2/inode.c    |    2 +
>  fs/ext3/inode.c    |    3 ++
>  include/linux/fs.h |    7 ++++
>  mm/filemap.c       |   95 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 107 insertions(+), 0 deletions(-)
> 
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 0079b2c..b3e4b50 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -695,6 +695,7 @@ const struct address_space_operations ext2_aops = {
>  	.direct_IO		= ext2_direct_IO,
>  	.writepages		= ext2_writepages,
>  	.migratepage		= buffer_migrate_page,
> +	.write_one_page		= generic_file_buffered_write_one_page,
>  };
>  
>  const struct address_space_operations ext2_aops_xip = {
> @@ -713,6 +714,7 @@ const struct address_space_operations ext2_nobh_aops = {
>  	.direct_IO		= ext2_direct_IO,
>  	.writepages		= ext2_writepages,
>  	.migratepage		= buffer_migrate_page,
> +	.write_one_page		= generic_file_buffered_write_one_page,
>  };
>  
>  /*
> diff --git a/fs/ext3/inode.c b/fs/ext3/inode.c
> index de4e316..93809eb 100644
> --- a/fs/ext3/inode.c
> +++ b/fs/ext3/inode.c
> @@ -1713,6 +1713,7 @@ static const struct address_space_operations ext3_ordered_aops = {
>  	.releasepage	= ext3_releasepage,
>  	.direct_IO	= ext3_direct_IO,
>  	.migratepage	= buffer_migrate_page,
> +	.write_one_page	= generic_file_buffered_write_one_page,
>  };
>  
>  static const struct address_space_operations ext3_writeback_aops = {
> @@ -1727,6 +1728,7 @@ static const struct address_space_operations ext3_writeback_aops = {
>  	.releasepage	= ext3_releasepage,
>  	.direct_IO	= ext3_direct_IO,
>  	.migratepage	= buffer_migrate_page,
> +	.write_one_page	= generic_file_buffered_write_one_page,
>  };
>  
>  static const struct address_space_operations ext3_journalled_aops = {
> @@ -1740,6 +1742,7 @@ static const struct address_space_operations ext3_journalled_aops = {
>  	.bmap		= ext3_bmap,
>  	.invalidatepage	= ext3_invalidatepage,
>  	.releasepage	= ext3_releasepage,
> +	.write_one_page	= generic_file_buffered_write_one_page,
>  };
>  
>  void ext3_set_aops(struct inode *inode)
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index bf35441..38f67ac 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -434,6 +434,11 @@ struct address_space_operations {
>  	int (*migratepage) (struct address_space *,
>  			struct page *, struct page *);
>  	int (*launder_page) (struct page *);
> +	/* write the contents of the source page over the page at the specified
> +	 * index in the target address space (the source page does not need to
> +	 * be related to the target address space) */
> +	int (*write_one_page)(struct address_space *, pgoff_t, struct page *);
> +
>  };
>  
>  struct backing_dev_info;
> @@ -1669,6 +1674,8 @@ extern ssize_t generic_file_direct_write(struct kiocb *, const struct iovec *,
>  		unsigned long *, loff_t, loff_t *, size_t, size_t);
>  extern ssize_t generic_file_buffered_write(struct kiocb *, const struct iovec *,
>  		unsigned long, loff_t, loff_t *, size_t, ssize_t);
> +extern int generic_file_buffered_write_one_page(struct address_space *,
> +						pgoff_t, struct page *);
>  extern ssize_t do_sync_read(struct file *filp, char __user *buf, size_t len, loff_t *ppos);
>  extern ssize_t do_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos);
>  extern void do_generic_mapping_read(struct address_space *mapping,
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 3a61923..21aeee9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2016,6 +2016,101 @@ zero_length_segment:
>  }
>  EXPORT_SYMBOL(generic_file_buffered_write);
>  
> +/**
> + * generic_file_buffered_write_one_page - Write a single page of data to an
> + *	inode
> + * @mapping - The address space of the target inode
> + * @index - The target page in the target inode to fill
> + * @source - The data to write into the target page
> + *
> + * Write the data from the source page to the page in the nominated address
> + * space at the @index specified.  Note that the file will not be extended if
> + * the page crosses the EOF marker, in which case only the first part of the
> + * page will be written.
> + *
> + * The @source page does not need to have any association with the file or the
> + * target page offset.
> + */
> +int generic_file_buffered_write_one_page(struct address_space *mapping,
> +					 pgoff_t index,
> +					 struct page *source)
> +{
> +	const struct address_space_operations *a_ops = mapping->a_ops;
> +	struct pagevec	lru_pvec;
> +	struct page *page, *cached_page = NULL;
> +	unsigned psize;
> +	loff_t isize;
> +	long status = 0;
> +
> +	pagevec_init(&lru_pvec, 0);
> +
> +	page = __grab_cache_page(mapping, index, &cached_page, &lru_pvec);
> +	if (!page) {
> +		BUG_ON(cached_page);
> +		return -ENOMEM;
> +	}
> +
> +	psize = PAGE_CACHE_SIZE;
> +	isize = i_size_read(mapping->host);
> +	if ((isize >> PAGE_SHIFT) == index)
> +		psize = isize & (PAGE_SIZE - 1);
> +
> +	status = a_ops->prepare_write(NULL, page, 0, psize);
> +	if (unlikely(status)) {
> +		isize = i_size_read(mapping->host);
> +
> +		if (status != AOP_TRUNCATED_PAGE)
> +			unlock_page(page);
> +		page_cache_release(page);
> +		if (status == AOP_TRUNCATED_PAGE)
> +			goto sync;
> +
> +		/* prepare_write() may have instantiated a few blocks outside
> +		 * i_size.  Trim these off again.
> +		 */
> +		if ((1ULL << (index + 1)) > isize)
> +			vmtruncate(mapping->host, isize);
> +		goto sync;
> +	}
> +
> +	copy_highpage(page, source);
> +	flush_dcache_page(page);
> +
> +	status = a_ops->commit_write(NULL, page, 0, psize);
> +	if (status == AOP_TRUNCATED_PAGE) {
> +		page_cache_release(page);
> +		goto sync;
> +	}
> +
> +	if (status > 0)
> +		status = 0;
> +
> +	unlock_page(page);
> +	mark_page_accessed(page);
> +	page_cache_release(page);
> +	if (status < 0)
> +		return status;
> +
> +	balance_dirty_pages_ratelimited(mapping);
> +	cond_resched();
> +
> +sync:
> +	if (cached_page)
> +		page_cache_release(cached_page);
> +
> +	/* the caller must handle O_SYNC themselves, but we handle S_SYNC and
> +	 * MS_SYNCHRONOUS here */
> +	if (unlikely(IS_SYNC(mapping->host)) && !a_ops->writepage)
> +		status = generic_osync_inode(mapping->host, mapping,
> +					     OSYNC_METADATA | OSYNC_DATA);
> +
> +	/* the caller must handle O_DIRECT for themselves */
> +
> +	pagevec_lru_add(&lru_pvec);
> +	return status;
> +}
> +EXPORT_SYMBOL(generic_file_buffered_write_one_page);
> +
>  static ssize_t
>  __generic_file_aio_write_nolock(struct kiocb *iocb, const struct iovec *iov,
>  				unsigned long nr_segs, loff_t *ppos)