Re: [PATCH v11 05/21] vfs,ext2: Introduce IS_DAX(inode)

[Mathieu Desnoyers]

On 25-Sep-2014 04:33:22 PM, Matthew Wilcox wrote:
> Use an inode flag to tag inodes which should avoid using the page cache.
> Convert ext2 to use it instead of mapping_is_xip().  Prevent I/Os to
> files tagged with the DAX flag from falling back to buffered I/O.

I agree that DAX enabled FS should not silently fallback to buffered
I/O, since it would void some guarantees about persistency of data that
has been written to a DAX mmap()'d region.

> 
> Signed-off-by: Matthew Wilcox <matthew.r.wilcox@xxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

> ---
>  fs/ext2/inode.c    |  9 ++++++---
>  fs/ext2/xip.h      |  2 --
>  include/linux/fs.h |  6 ++++++
>  mm/filemap.c       | 19 ++++++++++++-------
>  4 files changed, 24 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/ext2/inode.c b/fs/ext2/inode.c
> index 36d35c3..0cb0448 100644
> --- a/fs/ext2/inode.c
> +++ b/fs/ext2/inode.c
> @@ -731,7 +731,7 @@ static int ext2_get_blocks(struct inode *inode,
>  		goto cleanup;
>  	}
>  
> -	if (ext2_use_xip(inode->i_sb)) {
> +	if (IS_DAX(inode)) {
>  		/*
>  		 * we need to clear the block
>  		 */
> @@ -1201,7 +1201,7 @@ static int ext2_setsize(struct inode *inode, loff_t newsize)
>  
>  	inode_dio_wait(inode);
>  
> -	if (mapping_is_xip(inode->i_mapping))
> +	if (IS_DAX(inode))
>  		error = xip_truncate_page(inode->i_mapping, newsize);
>  	else if (test_opt(inode->i_sb, NOBH))
>  		error = nobh_truncate_page(inode->i_mapping,
> @@ -1273,7 +1273,8 @@ void ext2_set_inode_flags(struct inode *inode)
>  {
>  	unsigned int flags = EXT2_I(inode)->i_flags;
>  
> -	inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
> +	inode->i_flags &= ~(S_SYNC | S_APPEND | S_IMMUTABLE | S_NOATIME |
> +				S_DIRSYNC | S_DAX);
>  	if (flags & EXT2_SYNC_FL)
>  		inode->i_flags |= S_SYNC;
>  	if (flags & EXT2_APPEND_FL)
> @@ -1284,6 +1285,8 @@ void ext2_set_inode_flags(struct inode *inode)
>  		inode->i_flags |= S_NOATIME;
>  	if (flags & EXT2_DIRSYNC_FL)
>  		inode->i_flags |= S_DIRSYNC;
> +	if (test_opt(inode->i_sb, XIP))
> +		inode->i_flags |= S_DAX;
>  }
>  
>  /* Propagate flags from i_flags to EXT2_I(inode)->i_flags */
> diff --git a/fs/ext2/xip.h b/fs/ext2/xip.h
> index 18b34d2..29be737 100644
> --- a/fs/ext2/xip.h
> +++ b/fs/ext2/xip.h
> @@ -16,9 +16,7 @@ static inline int ext2_use_xip (struct super_block *sb)
>  }
>  int ext2_get_xip_mem(struct address_space *, pgoff_t, int,
>  				void **, unsigned long *);
> -#define mapping_is_xip(map) unlikely(map->a_ops->get_xip_mem)
>  #else
> -#define mapping_is_xip(map)			0
>  #define ext2_xip_verify_sb(sb)			do { } while (0)
>  #define ext2_use_xip(sb)			0
>  #define ext2_clear_xip_target(inode, chain)	0
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 9418772..e99e5c4 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1605,6 +1605,7 @@ struct super_operations {
>  #define S_IMA		1024	/* Inode has an associated IMA struct */
>  #define S_AUTOMOUNT	2048	/* Automount/referral quasi-directory */
>  #define S_NOSEC		4096	/* no suid or xattr security attributes */
> +#define S_DAX		8192	/* Direct Access, avoiding the page cache */
>  
>  /*
>   * Note that nosuid etc flags are inode-specific: setting some file-system
> @@ -1642,6 +1643,11 @@ struct super_operations {
>  #define IS_IMA(inode)		((inode)->i_flags & S_IMA)
>  #define IS_AUTOMOUNT(inode)	((inode)->i_flags & S_AUTOMOUNT)
>  #define IS_NOSEC(inode)		((inode)->i_flags & S_NOSEC)
> +#ifdef CONFIG_FS_XIP
> +#define IS_DAX(inode)		((inode)->i_flags & S_DAX)
> +#else
> +#define IS_DAX(inode)		0
> +#endif
>  
>  /*
>   * Inode state bits.  Protected by inode->i_lock
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 90effcd..fec4db9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1718,9 +1718,11 @@ generic_file_read_iter(struct kiocb *iocb, struct iov_iter *iter)
>  		 * we've already read everything we wanted to, or if
>  		 * there was a short read because we hit EOF, go ahead
>  		 * and return.  Otherwise fallthrough to buffered io for
> -		 * the rest of the read.
> +		 * the rest of the read.  Buffered reads will not work for
> +		 * DAX files, so don't bother trying.
>  		 */
> -		if (retval < 0 || !iov_iter_count(iter) || *ppos >= size) {
> +		if (retval < 0 || !iov_iter_count(iter) || *ppos >= size ||
> +		    IS_DAX(inode)) {
>  			file_accessed(file);
>  			goto out;
>  		}
> @@ -2584,13 +2586,16 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>  		loff_t endbyte;
>  
>  		written = generic_file_direct_write(iocb, from, pos);
> -		if (written < 0 || written == count)
> -			goto out;
> -
>  		/*
> -		 * direct-io write to a hole: fall through to buffered I/O
> -		 * for completing the rest of the request.
> +		 * If the write stopped short of completing, fall back to
> +		 * buffered writes.  Some filesystems do this for writes to
> +		 * holes, for example.  For DAX files, a buffered write will
> +		 * not succeed (even if it did, DAX does not handle dirty
> +		 * page-cache pages correctly).
>  		 */
> +		if (written < 0 || written == count || IS_DAX(inode))
> +			goto out;
> +
>  		pos += written;
>  		count -= written;
>  
> -- 
> 2.1.0
> 
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> 

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Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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