On Mon, Nov 28, 2016 at 02:09:36PM -0500, Brian Foster wrote:
> The XFS reflink implementation adds a copy-on-write inode fork to track
> newly allocated extents used to replace shared blocks on write. While,
> in principle, these extents are tracked by the cow fork temporarily,
> fragmentation avoidance mechanisms like the cowextsize hint and COW fork
> speculative preallocation allocate additional blocks outside of the
> range of the write. This means that blocks in the COW fork can linger
> for some time until written to and remapped to the data fork or reaped
> by the background cow fork reclaimer.
>
> To facilitate development and debugging, define and wire up a fiemap
> flag to query the cow fork extent list of an inode. Note that fiemap
> triggers writeback, which means all COW fork extents that are the target
> of I/O are remapped to the data fork as part of the query. As a result,
> the cow fork fiemap request returns only the blocks that have been
> allocated and not yet written to or reclaimed.
I don't think it's a good idea to expose implementation details through
a VFS level API. Since this is only for debugging purposes...
$ xfs_io -c 'bmap -c' /somefile
...should suffice and doesn't have the side effect of flushing dirty
file data.
FWIW, all the other fiemap implementations need to reject
FIEMAP_FLAG_COW if they don't know what that is.
--D
>
> Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> ---
> fs/xfs/xfs_iomap.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
> fs/xfs/xfs_iomap.h | 1 +
> fs/xfs/xfs_iops.c | 4 ++++
> include/uapi/linux/fiemap.h | 1 +
> 4 files changed, 51 insertions(+)
>
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 15a83813..4f46f49 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1159,3 +1159,48 @@ xfs_xattr_iomap_begin(
> struct iomap_ops xfs_xattr_iomap_ops = {
> .iomap_begin = xfs_xattr_iomap_begin,
> };
> +
> +static int
> +xfs_cow_iomap_begin(
> + struct inode *inode,
> + loff_t offset,
> + loff_t length,
> + unsigned flags,
> + struct iomap *iomap)
> +{
> + struct xfs_inode *ip = XFS_I(inode);
> + struct xfs_mount *mp = ip->i_mount;
> + xfs_fileoff_t offset_fsb = XFS_B_TO_FSBT(mp, offset);
> + xfs_fileoff_t end_fsb = XFS_B_TO_FSB(mp, offset + length);
> + struct xfs_bmbt_irec imap;
> + int error = 0;
> + int nimaps = 1;
> + unsigned lockmode;
> +
> + if (XFS_FORCED_SHUTDOWN(mp))
> + return -EIO;
> +
> + lockmode = xfs_ilock_data_map_shared(ip);
> +
> + if (!xfs_is_reflink_inode(ip)) {
> + error = -ENOENT;
> + goto out_unlock;
> + }
> +
> + error = xfs_bmapi_read(ip, offset_fsb, end_fsb - offset_fsb, &imap,
> + &nimaps, XFS_BMAPI_ENTIRE | XFS_BMAPI_COWFORK);
> +
> +out_unlock:
> + xfs_iunlock(ip, lockmode);
> +
> + if (!error) {
> + ASSERT(nimaps);
> + xfs_bmbt_to_iomap(ip, iomap, &imap);
> + }
> +
> + return error;
> +}
> +
> +struct iomap_ops xfs_cow_iomap_ops = {
> + .iomap_begin = xfs_cow_iomap_begin,
> +};
> diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h
> index 6d45cf0..69b62e4 100644
> --- a/fs/xfs/xfs_iomap.h
> +++ b/fs/xfs/xfs_iomap.h
> @@ -35,5 +35,6 @@ xfs_extlen_t xfs_eof_alignment(struct xfs_inode *ip, xfs_extlen_t extsize);
>
> extern struct iomap_ops xfs_iomap_ops;
> extern struct iomap_ops xfs_xattr_iomap_ops;
> +extern struct iomap_ops xfs_cow_iomap_ops;
>
> #endif /* __XFS_IOMAP_H__*/
> diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> index 405a65c..517eeed 100644
> --- a/fs/xfs/xfs_iops.c
> +++ b/fs/xfs/xfs_iops.c
> @@ -1043,6 +1043,10 @@ xfs_vn_fiemap(
> fieinfo->fi_flags &= ~FIEMAP_FLAG_XATTR;
> error = iomap_fiemap(inode, fieinfo, start, length,
> &xfs_xattr_iomap_ops);
> + } else if (fieinfo->fi_flags & FIEMAP_FLAG_COW) {
> + fieinfo->fi_flags &= ~FIEMAP_FLAG_COW;
> + error = iomap_fiemap(inode, fieinfo, start, length,
> + &xfs_cow_iomap_ops);
> } else {
> error = iomap_fiemap(inode, fieinfo, start, length,
> &xfs_iomap_ops);
> diff --git a/include/uapi/linux/fiemap.h b/include/uapi/linux/fiemap.h
> index 0c51d61..7014b4c 100644
> --- a/include/uapi/linux/fiemap.h
> +++ b/include/uapi/linux/fiemap.h
> @@ -41,6 +41,7 @@ struct fiemap {
> #define FIEMAP_FLAG_SYNC 0x00000001 /* sync file data before map */
> #define FIEMAP_FLAG_XATTR 0x00000002 /* map extended attribute tree */
> #define FIEMAP_FLAG_CACHE 0x00000004 /* request caching of the extents */
> +#define FIEMAP_FLAG_COW 0x00000010 /* map cow fork extents */
>
> #define FIEMAP_FLAGS_COMPAT (FIEMAP_FLAG_SYNC | FIEMAP_FLAG_XATTR)
>
> --
> 2.7.4
>
> --
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