[PATCH] Btrfs: fix incorrect compression ratio detection

[PATCH] Btrfs: fix incorrect compression ratio detection

[Wang Shilong]

Steps to reproduce:
 # mkfs.btrfs -f /dev/sdb
 # mount -t btrfs /dev/sdb /mnt -o compress=lzo
 # dd if=/dev/zero of=/mnt/data bs=$((33*4096)) count=1

after previous steps, inode will be detected as bad compression ratio,
and NOCOMPRESS flag will be set for that inode.

Reason is that compress have a max limit pages every time(128K), if a
132k write in, it will be splitted into two write(128k+4k), this bug
is a leftover for commit 68bb462d42a(Btrfs: don't compress for a small write)

Fix this problem by checking every time before compression, if it is a
small write(<=blocksize), we bail out and fall into nocompression directly.

Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>
---
 fs/btrfs/inode.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 344a322..b78e90a 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -411,14 +411,6 @@ static noinline int compress_file_range(struct inode *inode,
 	    (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
 		btrfs_add_inode_defrag(NULL, inode);
 
-	/*
-	 * skip compression for a small file range(<=blocksize) that
-	 * isn't an inline extent, since it dosen't save disk space at all.
-	 */
-	if ((end - start + 1) <= blocksize &&
-	    (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
-		goto cleanup_and_bail_uncompressed;
-
 	actual_end = min_t(u64, isize, end + 1);
 again:
 	will_compress = 0;
@@ -440,6 +432,14 @@ again:
 
 	total_compressed = actual_end - start;
 
+	/*
+	 * skip compression for a small file range(<=blocksize) that
+	 * isn't an inline extent, since it dosen't save disk space at all.
+	 */
+	if (total_compressed <= blocksize &&
+	   (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
+		goto cleanup_and_bail_uncompressed;
+
 	/* we want to make sure that amount of ram required to uncompress
 	 * an extent is reasonable, so we limit the total size in ram
 	 * of a compressed extent to 128k.  This is a crucial number
-- 
1.8.3.1

Re: [PATCH] Btrfs: fix incorrect compression ratio detection

[Miao Xie]

On Tue, 7 Oct 2014 18:44:35 -0400, Wang Shilong wrote:
> Steps to reproduce:
>  # mkfs.btrfs -f /dev/sdb
>  # mount -t btrfs /dev/sdb /mnt -o compress=lzo
>  # dd if=/dev/zero of=/mnt/data bs=$((33*4096)) count=1
> 
> after previous steps, inode will be detected as bad compression ratio,
> and NOCOMPRESS flag will be set for that inode.
> 
> Reason is that compress have a max limit pages every time(128K), if a
> 132k write in, it will be splitted into two write(128k+4k), this bug
> is a leftover for commit 68bb462d42a(Btrfs: don't compress for a small write)
> 
> Fix this problem by checking every time before compression, if it is a
> small write(<=blocksize), we bail out and fall into nocompression directly.
> 
> Signed-off-by: Wang Shilong <wangshilong1991@gmail.com>

Looks good.

Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>

> ---
>  fs/btrfs/inode.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
> index 344a322..b78e90a 100644
> --- a/fs/btrfs/inode.c
> +++ b/fs/btrfs/inode.c
> @@ -411,14 +411,6 @@ static noinline int compress_file_range(struct inode *inode,
>  	    (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
>  		btrfs_add_inode_defrag(NULL, inode);
>  
> -	/*
> -	 * skip compression for a small file range(<=blocksize) that
> -	 * isn't an inline extent, since it dosen't save disk space at all.
> -	 */
> -	if ((end - start + 1) <= blocksize &&
> -	    (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
> -		goto cleanup_and_bail_uncompressed;
> -
>  	actual_end = min_t(u64, isize, end + 1);
>  again:
>  	will_compress = 0;
> @@ -440,6 +432,14 @@ again:
>  
>  	total_compressed = actual_end - start;
>  
> +	/*
> +	 * skip compression for a small file range(<=blocksize) that
> +	 * isn't an inline extent, since it dosen't save disk space at all.
> +	 */
> +	if (total_compressed <= blocksize &&
> +	   (start > 0 || end + 1 < BTRFS_I(inode)->disk_i_size))
> +		goto cleanup_and_bail_uncompressed;
> +
>  	/* we want to make sure that amount of ram required to uncompress
>  	 * an extent is reasonable, so we limit the total size in ram
>  	 * of a compressed extent to 128k.  This is a crucial number
>