On 2018/8/28 下午1:54, Qu Wenruo wrote: > Due to the limitation of btrfs_cross_ref_exist(), run_delalloc_nocow() > can still fall back to CoW even only (unrelated) part of the > preallocated extent is shared. > > This makes the follow case to do unnecessary CoW: > > # xfs_io -f -c "falloc 0 2M" $mnt/file > # xfs_io -c "pwrite 0 1M" $mnt/file > # xfs_io -c "reflink $mnt/file 1M 4M 1M" $mnt/file > # sync > > The pwrite will still be CoWed, since at writeback time, the > preallocated extent is already shared, btrfs_cross_ref_exist() will > return 1 and make run_delalloc_nocow() fall back to cow_file_range(). > > This is definitely an overkilling workaround, but this should be the > simplest way without further screwing up already complex NOCOW routine.
Err, this is not even a working workaround.
It could still lead to bytes_may_use underflow as long as
btrfs_cross_ref_exist() could return 1 for partly shared prealloc extent.
So please ignore this patch.
Thanks,
Qu
>
> Signed-off-by: Qu Wenruo <w...@suse.com>
> ---
> fs/btrfs/ctree.h | 1 +
> fs/btrfs/file.c | 4 ++--
> fs/btrfs/ioctl.c | 21 +++++++++++++++++++++
> 3 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index 53af9f5253f4..ddacc41ff124 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -3228,6 +3228,7 @@ int btrfs_add_inode_defrag(struct btrfs_trans_handle
> *trans,
> struct btrfs_inode *inode);
> int btrfs_run_defrag_inodes(struct btrfs_fs_info *fs_info);
> void btrfs_cleanup_defrag_inodes(struct btrfs_fs_info *fs_info);
> +int btrfs_start_ordered_ops(struct inode *inode, loff_t start, loff_t end);
> int btrfs_sync_file(struct file *file, loff_t start, loff_t end, int
> datasync);
> void btrfs_drop_extent_cache(struct btrfs_inode *inode, u64 start, u64 end,
> int skip_pinned);
> diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
> index 2be00e873e92..118bfd019c6c 100644
> --- a/fs/btrfs/file.c
> +++ b/fs/btrfs/file.c
> @@ -1999,7 +1999,7 @@ int btrfs_release_file(struct inode *inode, struct file
> *filp)
> return 0;
> }
>
> -static int start_ordered_ops(struct inode *inode, loff_t start, loff_t end)
> +int btrfs_start_ordered_ops(struct inode *inode, loff_t start, loff_t end)
> {
> int ret;
> struct blk_plug plug;
> @@ -2056,7 +2056,7 @@ int btrfs_sync_file(struct file *file, loff_t start,
> loff_t end, int datasync)
> * multi-task, and make the performance up. See
> * btrfs_wait_ordered_range for an explanation of the ASYNC check.
> */
> - ret = start_ordered_ops(inode, start, end);
> + ret = btrfs_start_ordered_ops(inode, start, end);
> if (ret)
> goto out;
>
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 63600dc2ac4c..866979f530bc 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -4266,6 +4266,27 @@ static noinline int btrfs_clone_files(struct file
> *file, struct file *file_src,
> goto out_unlock;
> }
>
> + /*
> + * btrfs_cross_ref_exist() only does check at extent level,
> + * we could cause unexpected NOCOW write to be COWed.
> + * E.g.:
> + * falloc 0 2M file1
> + * pwrite 0 1M file1 (at this point it should go NOCOW)
> + * reflink src=file1 srcoff=1M dst=file1 dstoff=4M len=1M
> + * sync
> + *
> + * In above case, due to the preallocated extent is shared
> + * the data at 0~1M can't go NOCOW.
> + *
> + * So flush the whole src inode to avoid any unneeded CoW.
> + */
> + ret = btrfs_start_ordered_ops(src, 0, -1);
> + if (ret < 0)
> + goto out_unlock;
> + ret = btrfs_wait_ordered_range(src, 0, -1);
> + if (ret < 0)
> + goto out_unlock;
> +
> /*
> * Lock the target range too. Right after we replace the file extent
> * items in the fs tree (which now point to the cloned data), we might
>
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