erofs: free pclusters if no cached folio is attached

Once a pcluster is fully decompressed and there are no attached cached
folios, its corresponding `struct z_erofs_pcluster` will be freed. This
will significantly reduce the frequency of calls to erofs_shrink_scan()
and the memory allocated for `struct z_erofs_pcluster`.

The tables below show approximately a 96% reduction in the calls to
erofs_shrink_scan() and in the memory allocated for `struct
z_erofs_pcluster` after applying this patch. The results were obtained
by performing a test to copy a 4.1GB partition on ARM64 Android devices
running the 6.6 kernel with an 8-core CPU and 12GB of memory.

1. The reduction in calls to erofs_shrink_scan():
+-----------------+-----------+----------+---------+
|                 | w/o patch | w/ patch |  diff   |
+-----------------+-----------+----------+---------+
| Average (times) |   11390   |   390    | -96.57% |
+-----------------+-----------+----------+---------+

2. The reduction in memory released by erofs_shrink_scan():
+-----------------+-----------+----------+---------+
|                 | w/o patch | w/ patch |  diff   |
+-----------------+-----------+----------+---------+
| Average (Byte)  | 133612656 | 4434552  | -96.68% |
+-----------------+-----------+----------+---------+

Signed-off-by: Chunhai Guo <[email protected]>
Reviewed-by: Gao Xiang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Gao Xiang <[email protected]>

Author: Chunhai Guo

fs/erofs/zdata.c

@@ -885,22 +885,19 @@ static void z_erofs_rcu_callback(struct rcu_head *head)
 			struct z_erofs_pcluster, rcu));
 }
 
-static bool erofs_try_to_release_pcluster(struct erofs_sb_info *sbi,
+static bool __erofs_try_to_release_pcluster(struct erofs_sb_info *sbi,
 					  struct z_erofs_pcluster *pcl)
 {
-	int free = false;
-
-	spin_lock(&pcl->lockref.lock);
 	if (pcl->lockref.count)
-		goto out;
+		return false;
 
 	/*
 	 * Note that all cached folios should be detached before deleted from
 	 * the XArray.  Otherwise some folios could be still attached to the
 	 * orphan old pcluster when the new one is available in the tree.
 	 */
 	if (erofs_try_to_free_all_cached_folios(sbi, pcl))
-		goto out;
+		return false;
 
 	/*
 	 * It's impossible to fail after the pcluster is freezed, but in order
@@ -909,8 +906,16 @@ static bool erofs_try_to_release_pcluster(struct erofs_sb_info *sbi,
 	DBG_BUGON(__xa_erase(&sbi->managed_pslots, pcl->index) != pcl);
 
 	lockref_mark_dead(&pcl->lockref);
-	free = true;
-out:
+	return true;
+}
+
+static bool erofs_try_to_release_pcluster(struct erofs_sb_info *sbi,
+					  struct z_erofs_pcluster *pcl)
+{
+	bool free;
+
+	spin_lock(&pcl->lockref.lock);
+	free = __erofs_try_to_release_pcluster(sbi, pcl);
 	spin_unlock(&pcl->lockref.lock);
 	if (free) {
 		atomic_long_dec(&erofs_global_shrink_cnt);
@@ -942,16 +947,25 @@ unsigned long z_erofs_shrink_scan(struct erofs_sb_info *sbi,
 	return freed;
 }
 
-static void z_erofs_put_pcluster(struct z_erofs_pcluster *pcl)
+static void z_erofs_put_pcluster(struct erofs_sb_info *sbi,
+		struct z_erofs_pcluster *pcl, bool try_free)
 {
+	bool free = false;
+
 	if (lockref_put_or_lock(&pcl->lockref))
 		return;
 
 	DBG_BUGON(__lockref_is_dead(&pcl->lockref));
-	if (pcl->lockref.count == 1)
-		atomic_long_inc(&erofs_global_shrink_cnt);
-	--pcl->lockref.count;
+	if (!--pcl->lockref.count) {
+		if (try_free && xa_trylock(&sbi->managed_pslots)) {
+			free = __erofs_try_to_release_pcluster(sbi, pcl);
+			xa_unlock(&sbi->managed_pslots);
+		}
+		atomic_long_add(!free, &erofs_global_shrink_cnt);
+	}
 	spin_unlock(&pcl->lockref.lock);
+	if (free)
+		call_rcu(&pcl->rcu, z_erofs_rcu_callback);
 }
 
 static void z_erofs_pcluster_end(struct z_erofs_decompress_frontend *fe)
@@ -972,7 +986,7 @@ static void z_erofs_pcluster_end(struct z_erofs_decompress_frontend *fe)
 	 * any longer if the pcluster isn't hosted by ourselves.
 	 */
 	if (fe->mode < Z_EROFS_PCLUSTER_FOLLOWED_NOINPLACE)
-		z_erofs_put_pcluster(pcl);
+		z_erofs_put_pcluster(EROFS_I_SB(fe->inode), pcl, false);
 
 	fe->pcl = NULL;
 }
@@ -1274,6 +1288,7 @@ static int z_erofs_decompress_pcluster(struct z_erofs_decompress_backend *be,
 	int i, j, jtop, err2;
 	struct page *page;
 	bool overlapped;
+	bool try_free = true;
 
 	mutex_lock(&pcl->lock);
 	be->nr_pages = PAGE_ALIGN(pcl->length + pcl->pageofs_out) >> PAGE_SHIFT;
@@ -1331,9 +1346,12 @@ static int z_erofs_decompress_pcluster(struct z_erofs_decompress_backend *be,
 		/* managed folios are still left in compressed_bvecs[] */
 		for (i = 0; i < pclusterpages; ++i) {
 			page = be->compressed_pages[i];
-			if (!page ||
-			    erofs_folio_is_managed(sbi, page_folio(page)))
+			if (!page)
 				continue;
+			if (erofs_folio_is_managed(sbi, page_folio(page))) {
+				try_free = false;
+				continue;
+			}
 			(void)z_erofs_put_shortlivedpage(be->pagepool, page);
 			WRITE_ONCE(pcl->compressed_bvecs[i].page, NULL);
 		}
@@ -1379,6 +1397,11 @@ static int z_erofs_decompress_pcluster(struct z_erofs_decompress_backend *be,
 	/* pcluster lock MUST be taken before the following line */
 	WRITE_ONCE(pcl->next, Z_EROFS_PCLUSTER_NIL);
 	mutex_unlock(&pcl->lock);
+
+	if (z_erofs_is_inline_pcluster(pcl))
+		z_erofs_free_pcluster(pcl);
+	else
+		z_erofs_put_pcluster(sbi, pcl, try_free);
 	return err;
 }
 
@@ -1401,10 +1424,6 @@ static int z_erofs_decompress_queue(const struct z_erofs_decompressqueue *io,
 		owned = READ_ONCE(be.pcl->next);
 
 		err = z_erofs_decompress_pcluster(&be, err) ?: err;
-		if (z_erofs_is_inline_pcluster(be.pcl))
-			z_erofs_free_pcluster(be.pcl);
-		else
-			z_erofs_put_pcluster(be.pcl);
 	}
 	return err;
 }