FAILED: patch "[PATCH] ext4: handle wraparound when searching for blocks for" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x bb81702370fad22c06ca12b6e1648754dbc37e0f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026033045-backlands-hunk-0977@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From bb81702370fad22c06ca12b6e1648754dbc37e0f Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <[email protected]>
Date: Thu, 26 Mar 2026 00:58:34 -0400
Subject: [PATCH] ext4: handle wraparound when searching for blocks for
indirect mapped blocks

Commit 4865c768b563 ("ext4: always allocate blocks only from groups inode can use") restricts what blocks will be allocated for indirect block based files to block numbers that fit within 32-bit block numbers.

However, when using a review bot running on the latest Gemini LLM to check this commit when backporting into an LTS based kernel, it raised this concern:

If ac->ac_g_ex.fe_group is >= ngroups (for instance, if the goal group was populated via stream allocation from s_mb_last_groups), then start will be >= ngroups.

Does this allow allocating blocks beyond the 32-bit limit for indirect block mapped files? The commit message mentions that ext4_mb_scan_groups_linear() takes care to not select unsupported groups. However, its loop uses group = *start, and the very first iteration will call ext4_mb_scan_group() with this unsupported group because next_linear_group() is only called at the end of the iteration.

After reviewing the code paths involved and considering the LLM review, I determined that this can happen when there is a file system where some files/directories are extent-mapped and others are indirect-block mapped. To address this, add a safety clamp in ext4_mb_scan_groups().

index bb6faebf9b6d..cb2bd87c355c 100644
@@ -1199,6 +1199,8 @@ static int ext4_mb_scan_groups(struct ext4_allocation_context *ac)
 
 	/* searching for the right group start from the goal value specified */
 	start = ac->ac_g_ex.fe_group;
+	if (start >= ngroups)
+		start = 0;
 	ac->ac_prefetch_grp = start;
 	ac->ac_prefetch_nr = 0;