linux-ext4 - [PATCH 4/9] ext4: prevent stale extent cache entries caused by concurrent fiemap

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Message-ID: <20250423085257.122685-5-yi.zhang@huaweicloud.com>
Date: Wed, 23 Apr 2025 16:52:52 +0800
From: Zhang Yi <yi.zhang@...weicloud.com>
To: linux-ext4@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	tytso@....edu,
	adilger.kernel@...ger.ca,
	jack@...e.cz,
	yi.zhang@...wei.com,
	yi.zhang@...weicloud.com,
	libaokun1@...wei.com,
	yukuai3@...wei.com,
	yangerkun@...wei.com
Subject: [PATCH 4/9] ext4: prevent stale extent cache entries caused by concurrent fiemap

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

The ext4_fiemap() currently invokes ext4_ext_precache() and
iomap_fiemap() to preload the extent cache and query mapping information
without holding the inode's i_rwsem. This can result in stale extent
cache entries when competing with operations such as
ext4_collapse_range() which calls ext4_ext_remove_space() or
ext4_ext_shift_extents().

The problem arises when ext4_ext_remove_space() temporarily releases
i_data_sem due to insufficient journal credits. During this interval, a
concurrent ext4_fiemap() may cache extent entries that are about to be
deleted. As a result, these cached entries become stale and inconsistent
with the actual extents.

Loading the extents cache without holding the inode's i_rwsem or the
mapping's invalidate_lock is not permitted besides during the writeback.
Fix this by holding the i_rwsem in ext4_fiemap().

Signed-off-by: Zhang Yi <yi.zhang@...wei.com>
---
 fs/ext4/extents.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 8a5724b2dc51..3adf05fbdd59 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4963,10 +4963,11 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 {
 	int error = 0;

+	inode_lock_shared(inode);
 	if (fieinfo->fi_flags & FIEMAP_FLAG_CACHE) {
 		error = ext4_ext_precache(inode);
 		if (error)
-			return error;
+			goto unlock;
 		fieinfo->fi_flags &= ~FIEMAP_FLAG_CACHE;
 	}

@@ -4977,15 +4978,19 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	 */
 	error = ext4_fiemap_check_ranges(inode, start, &len);
 	if (error)
-		return error;
+		goto unlock;

 	if (fieinfo->fi_flags & FIEMAP_FLAG_XATTR) {
 		fieinfo->fi_flags &= ~FIEMAP_FLAG_XATTR;
-		return iomap_fiemap(inode, fieinfo, start, len,
-				    &ext4_iomap_xattr_ops);
+		error = iomap_fiemap(inode, fieinfo, start, len,
+				     &ext4_iomap_xattr_ops);
+	} else {
+		error = iomap_fiemap(inode, fieinfo, start, len,
+				     &ext4_iomap_report_ops);
 	}
-
-	return iomap_fiemap(inode, fieinfo, start, len, &ext4_iomap_report_ops);
+unlock:
+	inode_unlock_shared(inode);
+	return error;
 }

 int ext4_get_es_cache(struct inode *inode, struct fiemap_extent_info *fieinfo,
-- 
2.46.1