+ fat-fix-parent-link-count-when-replacing-a-sibling-directory.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: fat: fix parent link count when replacing a sibling directory
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fat-fix-parent-link-count-when-replacing-a-sibling-directory.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fat-fix-parent-link-count-when-replacing-a-sibling-directory.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Hungyu Lin <dennylin0707@gmail.com>
Subject: fat: fix parent link count when replacing a sibling directory
Date: Thu, 29 Oct 2026 20:23:52 +0000

vfat_rename() and do_msdos_rename() adjust parent link counts only when
updating the dotdot entry for a move between different parents. Replacing
an existing empty directory in the same parent also removes a
subdirectory, but leaves the parent's link count unchanged.

For example, renaming one of two sibling directories over the other
leaves the parent with four links instead of three. stat and ls report
the stale count, and find -links 3 does not match the parent.

Drop the parent's link count for this case after the source entry has
been removed successfully. Leave cross-parent renames, regular files
and the existing error paths unchanged.

As in rmdir, check that the parent's link count is at least three before
decrementing it. Report a filesystem error if corrupted directory entries
have left the count too low, rather than reducing it below two.

Link: https://lore.kernel.org/20261029202352.59890-1-dennylin0707@gmail.com
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Christian Brauner <brauner@kernel.org>
---

 fs/fat/namei_msdos.c |    8 ++++++++
 fs/fat/namei_vfat.c  |    8 ++++++++
 2 files changed, 16 insertions(+)

--- a/fs/fat/namei_msdos.c~fat-fix-parent-link-count-when-replacing-a-sibling-directory
+++ a/fs/fat/namei_msdos.c
@@ -565,6 +565,14 @@ static int do_msdos_rename(struct inode
 		(void)sync_inode_metadata(old_dir, 1);
 
 	if (new_inode) {
+		if (is_dir && old_dir == new_dir) {
+			if (old_dir->i_nlink >= 3)
+				drop_nlink(old_dir);
+			else
+				fat_fs_error(old_dir->i_sb,
+					     "parent dir link count too low (%u)",
+					     old_dir->i_nlink);
+		}
 		drop_nlink(new_inode);
 		if (is_dir)
 			drop_nlink(new_inode);
--- a/fs/fat/namei_vfat.c~fat-fix-parent-link-count-when-replacing-a-sibling-directory
+++ a/fs/fat/namei_vfat.c
@@ -1011,6 +1011,14 @@ static int vfat_rename(struct inode *old
 	vfat_update_dir_metadata(old_dir, &ts);
 
 	if (new_inode) {
+		if (is_dir && old_dir == new_dir) {
+			if (old_dir->i_nlink >= 3)
+				drop_nlink(old_dir);
+			else
+				fat_fs_error(old_dir->i_sb,
+					     "parent dir link count too low (%u)",
+					     old_dir->i_nlink);
+		}
 		drop_nlink(new_inode);
 		if (is_dir)
 			drop_nlink(new_inode);
_

Patches currently in -mm which might be from dennylin0707@gmail.com are

mm-page-writeback-avoid-32-bit-overflow-in-dirty-threshold-shares.patch
mm-migrate-untag-addresses-when-querying-page-nodes.patch
fat-fix-parent-link-count-when-replacing-a-sibling-directory.patch