[PATCH v2] erofs-utils: support adjust lz4 history window size

[Huang Jianan]

lz4 uses LZ4_DISTANCE_MAX to record history preservation. When
using rolling decompression, a block with a higher compression
ratio will cause a larger memory allocation (up to 64k). It may
cause a large resource burden in extreme cases on devices with
small memory and a large number of concurrent IOs. So appropriately
reducing this value can improve performance.

Decreasing this value will reduce the compression ratio (except
when input_size <LZ4_DISTANCE_MAX). But considering that erofs
currently only supports 4k output, reducing this value will not
significantly reduce the compression benefits.

Signed-off-by: Huang Jianan <huangjianan at oppo.com>
Signed-off-by: Guo Weichao <guoweichao at oppo.com>
---

changes since previous version
- change compr_alg to lz4_max_distance

 include/erofs/internal.h | 3 +++
 include/erofs_fs.h       | 3 ++-
 lib/compressor_lz4.c     | 5 +++++
 lib/compressor_lz4hc.c   | 3 +++
 mkfs/main.c              | 1 +
 5 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/include/erofs/internal.h b/include/erofs/internal.h
index ac5b270..9066910 100644
--- a/include/erofs/internal.h
+++ b/include/erofs/internal.h
@@ -79,6 +79,9 @@ struct erofs_sb_info {
 	u64 inos;
 
 	u8 uuid[16];
+
+	/* lz4 max distance */
+	u16 lz4_max_distance;
 };
 
 /* global sbi */
diff --git a/include/erofs_fs.h b/include/erofs_fs.h
index a69f179..f38cba2 100644
--- a/include/erofs_fs.h
+++ b/include/erofs_fs.h
@@ -41,7 +41,8 @@ struct erofs_super_block {
 	__u8 uuid[16];          /* 128-bit uuid for volume */
 	__u8 volume_name[16];   /* volume name */
 	__le32 feature_incompat;
-	__u8 reserved2[44];
+	__le16 lz4_max_distance;	/* lz4 max distance */
+	__u8 reserved2[42];
 };
 
 /*
diff --git a/lib/compressor_lz4.c b/lib/compressor_lz4.c
index 8540a0d..292d0f2 100644
--- a/lib/compressor_lz4.c
+++ b/lib/compressor_lz4.c
@@ -10,6 +10,10 @@
 #include "erofs/internal.h"
 #include "compressor.h"
 
+#ifndef LZ4_DISTANCE_MAX	/* history window size */
+#define LZ4_DISTANCE_MAX 65535	/* set to maximum value by default */
+#endif
+
 static int lz4_compress_destsize(struct erofs_compress *c,
 				 int compression_level,
 				 void *src, unsigned int *srcsize,
@@ -32,6 +36,7 @@ static int compressor_lz4_exit(struct erofs_compress *c)
 static int compressor_lz4_init(struct erofs_compress *c)
 {
 	c->alg = &erofs_compressor_lz4;
+	sbi.lz4_max_distance = LZ4_DISTANCE_MAX;
 	return 0;
 }
 
diff --git a/lib/compressor_lz4hc.c b/lib/compressor_lz4hc.c
index 6680563..00fa2c2 100644
--- a/lib/compressor_lz4hc.c
+++ b/lib/compressor_lz4hc.c
@@ -44,6 +44,9 @@ static int compressor_lz4hc_init(struct erofs_compress *c)
 	c->private_data = LZ4_createStreamHC();
 	if (!c->private_data)
 		return -ENOMEM;
+
+	sbi.lz4_max_distance = LZ4_DISTANCE_MAX;
+
 	return 0;
 }
 
diff --git a/mkfs/main.c b/mkfs/main.c
index d9c4c7f..4c9743d 100644
--- a/mkfs/main.c
+++ b/mkfs/main.c
@@ -299,6 +299,7 @@ int erofs_mkfs_update_super_block(struct erofs_buffer_head *bh,
 		.feature_incompat = cpu_to_le32(sbi.feature_incompat),
 		.feature_compat = cpu_to_le32(sbi.feature_compat &
 					      ~EROFS_FEATURE_COMPAT_SB_CHKSUM),
+		.lz4_max_distance = cpu_to_le16(sbi.lz4_max_distance),
 	};
 	const unsigned int sb_blksize =
 		round_up(EROFS_SUPER_END, EROFS_BLKSIZ);
-- 
2.25.1