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

[Huang Jianan]

From: huangjianan <huangjianan@xxxxxxxx>

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@xxxxxxxx>
Signed-off-by: Guo Weichao <guoweichao@xxxxxxxx>
---

changes since previous version
- change compr_alg to lz4_max_distance
- calculate lz4_max_distance_pages when reading super_block

 fs/erofs/decompressor.c | 12 ++++++++----
 fs/erofs/erofs_fs.h     |  3 ++-
 fs/erofs/internal.h     |  3 +++
 fs/erofs/super.c        |  5 +++++
 4 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/fs/erofs/decompressor.c b/fs/erofs/decompressor.c
index 1cb1ffd10569..fb2b4f1b8806 100644
--- a/fs/erofs/decompressor.c
+++ b/fs/erofs/decompressor.c
@@ -36,22 +36,26 @@ static int z_erofs_lz4_prepare_destpages(struct z_erofs_decompress_req *rq,
 	struct page *availables[LZ4_MAX_DISTANCE_PAGES] = { NULL };
 	unsigned long bounced[DIV_ROUND_UP(LZ4_MAX_DISTANCE_PAGES,
 					   BITS_PER_LONG)] = { 0 };
+	unsigned int lz4_max_distance_pages = LZ4_MAX_DISTANCE_PAGES;
 	void *kaddr = NULL;
 	unsigned int i, j, top;
 
+	if (EROFS_SB(rq->sb)->lz4_max_distance_pages)
+		lz4_max_distance_pages = EROFS_SB(rq->sb)->lz4_max_distance_pages;
+
 	top = 0;
 	for (i = j = 0; i < nr; ++i, ++j) {
 		struct page *const page = rq->out[i];
 		struct page *victim;
 
-		if (j >= LZ4_MAX_DISTANCE_PAGES)
+		if (j >= lz4_max_distance_pages)
 			j = 0;
 
 		/* 'valid' bounced can only be tested after a complete round */
 		if (test_bit(j, bounced)) {
-			DBG_BUGON(i < LZ4_MAX_DISTANCE_PAGES);
-			DBG_BUGON(top >= LZ4_MAX_DISTANCE_PAGES);
-			availables[top++] = rq->out[i - LZ4_MAX_DISTANCE_PAGES];
+			DBG_BUGON(i < lz4_max_distance_pages);
+			DBG_BUGON(top >= lz4_max_distance_pages);
+			availables[top++] = rq->out[i - lz4_max_distance_pages];
 		}
 
 		if (page) {
diff --git a/fs/erofs/erofs_fs.h b/fs/erofs/erofs_fs.h
index 9ad1615f4474..5eb37002b1a3 100644
--- a/fs/erofs/erofs_fs.h
+++ b/fs/erofs/erofs_fs.h
@@ -39,7 +39,8 @@ 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/fs/erofs/internal.h b/fs/erofs/internal.h
index 67a7ec945686..7457710a763a 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -70,6 +70,9 @@ struct erofs_sb_info {
 
 	/* pseudo inode to manage cached pages */
 	struct inode *managed_cache;
+
+	/* lz4 max distance pages */
+	u16 lz4_max_distance_pages;
 #endif	/* CONFIG_EROFS_FS_ZIP */
 	u32 blocks;
 	u32 meta_blkaddr;
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index d5a6b9b888a5..3a3d235de7cc 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -174,6 +174,11 @@ static int erofs_read_superblock(struct super_block *sb)
 	sbi->islotbits = ilog2(sizeof(struct erofs_inode_compact));
 	sbi->root_nid = le16_to_cpu(dsb->root_nid);
 	sbi->inos = le64_to_cpu(dsb->inos);
+#ifdef CONFIG_EROFS_FS_ZIP
+	if (dsb->lz4_max_distance)
+		sbi->lz4_max_distance_pages =
+			DIV_ROUND_UP(le16_to_cpu(dsb->lz4_max_distance), PAGE_SIZE) + 1;
+#endif
 
 	sbi->build_time = le64_to_cpu(dsb->build_time);
 	sbi->build_time_nsec = le32_to_cpu(dsb->build_time_nsec);
-- 
2.25.1