[PATCH 2/2] erofs: support to readahead dirent blocks in erofs_readdir()

[Chao Yu]

This patch supports to readahead more blocks in erofs_readdir(),
it can enhance performance in large direcotry.

readdir test in a large directory which contains 12000 sub-files.

		files_per_second
Before:		926385.54
After:		2380435.562

Signed-off-by: Chao Yu <chao at kernel.org>
---
 fs/erofs/dir.c      | 8 ++++++++
 fs/erofs/internal.h | 3 +++
 2 files changed, 11 insertions(+)

diff --git a/fs/erofs/dir.c b/fs/erofs/dir.c
index cff61c5a172b..04113851fc0f 100644
--- a/fs/erofs/dir.c
+++ b/fs/erofs/dir.c
@@ -47,8 +47,10 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
 	struct inode *dir = file_inode(f);
 	struct erofs_buf buf = __EROFS_BUF_INITIALIZER;
 	struct super_block *sb = dir->i_sb;
+	struct file_ra_state *ra = &f->f_ra;
 	unsigned long bsz = sb->s_blocksize;
 	unsigned int ofs = erofs_blkoff(sb, ctx->pos);
+	unsigned long nr_pages = DIV_ROUND_UP_POW2(dir->i_size, PAGE_SIZE);
 	int err = 0;
 	bool initial = true;
 
@@ -65,6 +67,12 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
 		}
 		cond_resched();
 
+		/* readahead blocks to enhance performance in large directory */
+		if (nr_pages - dbstart > 1 && !ra_has_index(ra, dbstart))
+			page_cache_sync_readahead(dir->i_mapping, ra, f,
+				dbstart, min(nr_pages - dbstart,
+				(pgoff_t)MAX_DIR_RA_PAGES));
+
 		de = erofs_bread(&buf, dbstart, true);
 		if (IS_ERR(de)) {
 			erofs_err(sb, "failed to readdir of logical block %llu of nid %llu",
diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index a32c03a80c70..ef9d1ee8c688 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -238,6 +238,9 @@ EROFS_FEATURE_FUNCS(xattr_filter, compat, COMPAT_XATTR_FILTER)
 #define EROFS_I_BL_XATTR_BIT	(BITS_PER_LONG - 1)
 #define EROFS_I_BL_Z_BIT	(BITS_PER_LONG - 2)
 
+/* maximum readahead pages of directory */
+#define MAX_DIR_RA_PAGES	4
+
 struct erofs_inode {
 	erofs_nid_t nid;
 
-- 
2.49.0