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Date: Thu, 16 Oct 2014 11:35:17 +0200
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Matthew Wilcox <matthew.r.wilcox@...el.com>
Cc: linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 05/21] vfs,ext2: Introduce IS_DAX(inode)
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@...el.com>
> Reviewed-by: Jan Kara <jack@...e.cz>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> ---
> 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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>
>
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
Key fingerprint: 2A0B 4ED9 15F2 D3FA 45F5 B162 1728 0A97 8118 6ACF
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