linux-ext4 - [RFC PATCH 03/18] ext4: correct the hole length returned by ext4_map

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Date: Thu, 23 Nov 2023 20:51:05 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: linux-ext4@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org,
	tytso@....edu,
	adilger.kernel@...ger.ca,
	jack@...e.cz,
	ritesh.list@...il.com,
	hch@...radead.org,
	djwong@...nel.org,
	yi.zhang@...wei.com,
	yi.zhang@...weicloud.com,
	chengzhihao1@...wei.com,
	yukuai3@...wei.com
Subject: [RFC PATCH 03/18] ext4: correct the hole length returned by ext4_map_blocks()

From: Zhang Yi <yi.zhang@...wei.com>

In ext4_map_blocks(), if we can't find a range of mapping in the
extents cache, we are calling ext4_ext_map_blocks() to search the real
path. But if the querying range was tail overlaped by a delayed extent,
we can't find it on the real extent path, so the returned hole length
could be larger than it really is.

      |          querying map          |
      v                                v
      |----------{-------------}{------|----------------}-----...
      ^          ^             ^^                       ^
      | uncached | hole extent ||     delayed extent    |

We have to adjust the mapping length to the next not hole extent's
lblk before searching the extent path.

Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
---
 fs/ext4/inode.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 4ce35f1c8b0a..94e7b8500878 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -479,6 +479,7 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		    struct ext4_map_blocks *map, int flags)
 {
 	struct extent_status es;
+	ext4_lblk_t next;
 	int retval;
 	int ret = 0;
 #ifdef ES_AGGRESSIVE_TEST
@@ -502,8 +503,10 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 		return -EFSCORRUPTED;
 
 	/* Lookup extent status tree firstly */
-	if (!(EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY) &&
-	    ext4_es_lookup_extent(inode, map->m_lblk, NULL, &es)) {
+	if (EXT4_SB(inode->i_sb)->s_mount_state & EXT4_FC_REPLAY)
+		goto uncached;
+
+	if (ext4_es_lookup_extent(inode, map->m_lblk, NULL, &es)) {
 		if (ext4_es_is_written(&es) || ext4_es_is_unwritten(&es)) {
 			map->m_pblk = ext4_es_pblock(&es) +
 					map->m_lblk - es.es_lblk;
@@ -532,6 +535,23 @@ int ext4_map_blocks(handle_t *handle, struct inode *inode,
 #endif
 		goto found;
 	}
+	/*
+	 * Not found, maybe a hole, need to adjust the map length before
+	 * seraching the real extent path. It can prevent incorrect hole
+	 * length returned if the following entries have delayed only
+	 * ones.
+	 */
+	if (!(flags & EXT4_GET_BLOCKS_CREATE) && es.es_lblk > map->m_lblk) {
+		next = es.es_lblk;
+		if (ext4_es_is_hole(&es))
+			next = ext4_es_skip_hole_extent(inode, map->m_lblk,
+							map->m_len);
+		retval = next - map->m_lblk;
+		if (map->m_len > retval)
+			map->m_len = retval;
+	}
+
+uncached:
 	/*
 	 * In the query cache no-wait mode, nothing we can do more if we
 	 * cannot find extent in the cache.
-- 
2.39.2