[PATCH] fsck.erofs: introduce rw-semaphore metadata cache PoC

[Nithurshen <nithurshen.dev@gmail.com>]

This PoC introduces a thread-safe metadata cache to reduce redundant I/O
and decompression overhead during fsck extraction. It directly addresses
the TODO in erofs_bread by modeling a bucketed, rw-semaphore protected
cache after the existing fragment cache implementation.

Baseline (LZ4HC 4K pclusters, Linux 6.7 tree):
Extraction time: 1.538s

With Meta Cache PoC:
Extraction time: 1.090s (~29% reduction)

Signed-off-by: Nithurshen <nithurshen.dev@gmail.com>
---
 lib/data.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 81 insertions(+), 3 deletions(-)

diff --git a/lib/data.c b/lib/data.c
index 6fd1389..bcd8d17 100644
--- a/lib/data.c
+++ b/lib/data.c
@@ -9,6 +9,35 @@
 #include "erofs/trace.h"
 #include "erofs/decompress.h"
 #include "liberofs_fragments.h"
+#include "erofs/lock.h"
+
+#define META_HASHSIZE		65536
+#define META_HASH(c)		((c) & (META_HASHSIZE - 1))
+
+struct erofs_meta_bucket {
+	struct list_head hash;
+	erofs_rwsem_t lock;
+};
+
+struct erofs_meta_item {
+	struct list_head list;
+	u64 key;
+	char *data;
+	int length;
+};
+
+static struct erofs_meta_bucket meta_bks[META_HASHSIZE];
+static bool meta_cache_inited = false;
+
+static void erofs_meta_cache_init(void)
+{
+	int i;
+	for (i = 0; i < META_HASHSIZE; ++i) {
+		init_list_head(&meta_bks[i].hash);
+		erofs_init_rwsem(&meta_bks[i].lock);
+	}
+	meta_cache_inited = true;
+}
 
 void *erofs_bread(struct erofs_buf *buf, erofs_off_t offset, bool need_kmap)
 {
@@ -500,7 +529,56 @@ static void *erofs_read_metadata_bdi(struct erofs_sb_info *sbi,
 void *erofs_read_metadata(struct erofs_sb_info *sbi, erofs_nid_t nid,
 			  erofs_off_t *offset, int *lengthp)
 {
+	u64 key = nid ? nid : *offset;
+	struct erofs_meta_bucket *bk;
+	struct erofs_meta_item *item;
+	void *buffer = NULL;
+
+	if (__erofs_unlikely(!meta_cache_inited))
+		erofs_meta_cache_init();
+
+	bk = &meta_bks[META_HASH(key)];
+
+	erofs_down_read(&bk->lock);
+	list_for_each_entry(item, &bk->hash, list) {
+		if (item->key == key) {
+			buffer = malloc(item->length);
+			if (buffer) {
+				memcpy(buffer, item->data, item->length);
+				*lengthp = item->length;
+				*offset = round_up(*offset, 4);
+				*offset += sizeof(__le16) + item->length;
+			}
+			break;
+		}
+	}
+	erofs_up_read(&bk->lock);
+
+	if (buffer)
+		return buffer;
+
 	if (nid)
-		return erofs_read_metadata_nid(sbi, nid, offset, lengthp);
-	return erofs_read_metadata_bdi(sbi, offset, lengthp);
-}
+		buffer = erofs_read_metadata_nid(sbi, nid, offset, lengthp);
+	else
+		buffer = erofs_read_metadata_bdi(sbi, offset, lengthp);
+
+	if (IS_ERR(buffer))
+		return buffer;
+
+	item = malloc(sizeof(*item));
+	if (item) {
+		item->key = key;
+		item->length = *lengthp;
+		item->data = malloc(*lengthp);
+		if (item->data) {
+			memcpy(item->data, buffer, *lengthp);
+			erofs_down_write(&bk->lock);
+			list_add_tail(&item->list, &bk->hash);
+			erofs_up_write(&bk->lock);
+		} else {
+			free(item);
+		}
+	}
+
+	return buffer;
+}
\ No newline at end of file
-- 
2.51.0