Merge tag 'mm-nonmm-stable-2026-04-15-04-20' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

Pull non-MM updates from Andrew Morton:

 - "pid: make sub-init creation retryable" (Oleg Nesterov)

   Make creation of init in a new namespace more robust by clearing away
   some historical cruft which is no longer needed. Also some
   documentation fixups

 - "selftests/fchmodat2: Error handling and general" (Mark Brown)

   Fix and a cleanup for the fchmodat2() syscall selftest

 - "lib: polynomial: Move to math/ and clean up" (Andy Shevchenko)

 - "hung_task: Provide runtime reset interface for hung task detector"
   (Aaron Tomlin)

   Give administrators the ability to zero out
   /proc/sys/kernel/hung_task_detect_count

 - "tools/getdelays: use the static UAPI headers from
   tools/include/uapi" (Thomas Weißschuh)

   Teach getdelays to use the in-kernel UAPI headers rather than the
   system-provided ones

 - "watchdog/hardlockup: Improvements to hardlockup" (Mayank Rungta)

   Several cleanups and fixups to the hardlockup detector code and its
   documentation

 - "lib/bch: fix undefined behavior from signed left-shifts" (Josh Law)

   A couple of small/theoretical fixes in the bch code

 - "ocfs2/dlm: fix two bugs in dlm_match_regions()" (Junrui Luo)

 - "cleanup the RAID5 XOR library" (Christoph Hellwig)

   A quite far-reaching cleanup to this code. I can't do better than to
   quote Christoph:

     "The XOR library used for the RAID5 parity is a bit of a mess right
      now. The main file sits in crypto/ despite not being cryptography
      and not using the crypto API, with the generic implementations
      sitting in include/asm-generic and the arch implementations
      sitting in an asm/ header in theory. The latter doesn't work for
      many cases, so architectures often build the code directly into
      the core kernel, or create another module for the architecture
      code.

      Change this to a single module in lib/ that also contains the
      architecture optimizations, similar to the library work Eric
      Biggers has done for the CRC and crypto libraries later. After
      that it changes to better calling conventions that allow for
      smarter architecture implementations (although none is contained
      here yet), and uses static_call to avoid indirection function call
      overhead"

 - "lib/list_sort: Clean up list_sort() scheduling workarounds"
   (Kuan-Wei Chiu)

   Clean up this library code by removing a hacky thing which was added
   for UBIFS, which UBIFS doesn't actually need

 - "Fix bugs in extract_iter_to_sg()" (Christian Ehrhardt)

   Fix a few bugs in the scatterlist code, add in-kernel tests for the
   now-fixed bugs and fix a leak in the test itself

 - "kdump: Enable LUKS-encrypted dump target support in ARM64 and
   PowerPC" (Coiby Xu)

   Enable support of the LUKS-encrypted device dump target on arm64 and
   powerpc

 - "ocfs2: consolidate extent list validation into block read callbacks"
   (Joseph Qi)

   Cleanup, simplify, and make more robust ocfs2's validation of extent
   list fields (Kernel test robot loves mounting corrupted fs images!)

* tag 'mm-nonmm-stable-2026-04-15-04-20' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (127 commits)
  ocfs2: validate group add input before caching
  ocfs2: validate bg_bits during freefrag scan
  ocfs2: fix listxattr handling when the buffer is full
  doc: watchdog: fix typos etc
  update Sean's email address
  ocfs2: use get_random_u32() where appropriate
  ocfs2: split transactions in dio completion to avoid credit exhaustion
  ocfs2: remove redundant l_next_free_rec check in __ocfs2_find_path()
  ocfs2: validate extent block list fields during block read
  ocfs2: remove empty extent list check in ocfs2_dx_dir_lookup_rec()
  ocfs2: validate dx_root extent list fields during block read
  ocfs2: fix use-after-free in ocfs2_fault() when VM_FAULT_RETRY
  ocfs2: handle invalid dinode in ocfs2_group_extend
  .get_maintainer.ignore: add Askar
  ocfs2: validate bg_list extent bounds in discontig groups
  checkpatch: exclude forward declarations of const structs
  tools/accounting: handle truncated taskstats netlink messages
  taskstats: set version in TGID exit notifications
  ocfs2/heartbeat: fix slot mapping rollback leaks on error paths
  arm64,ppc64le/kdump: pass dm-crypt keys to kdump kernel
  ...

2 files changed, 123 insertions, 26 deletions

diff --git a/Documentation/admin-guide/lockup-watchdogs.rst b/Documentation/admin-guide/lockup-watchdogs.rst
index 3e09284a8b9b..8f245f4a95b7 100644
--- a/Documentation/admin-guide/lockup-watchdogs.rst
+++ b/Documentation/admin-guide/lockup-watchdogs.rst
@@ -16,7 +16,7 @@ details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
 provided for this.
 
 A 'hardlockup' is defined as a bug that causes the CPU to loop in
-kernel mode for more than 10 seconds (see "Implementation" below for
+kernel mode for several seconds (see "Implementation" below for
 details), without letting other interrupts have a chance to run.
 Similarly to the softlockup case, the current stack trace is displayed
 upon detection and the system will stay locked up unless the default
@@ -30,39 +30,135 @@ timeout is set through the confusingly named "kernel.panic" sysctl),
 to cause the system to reboot automatically after a specified amount
 of time.
 
+Configuration
+=============
+
+A kernel knob is provided that allows administrators to configure
+this period. The "watchdog_thresh" parameter (default 10 seconds)
+controls the threshold. The right value for a particular environment
+is a trade-off between fast response to lockups and detection overhead.
+
 Implementation
 ==============
 
-The soft and hard lockup detectors are built on top of the hrtimer and
-perf subsystems, respectively. A direct consequence of this is that,
-in principle, they should work in any architecture where these
-subsystems are present.
-
-A periodic hrtimer runs to generate interrupts and kick the watchdog
-job. An NMI perf event is generated every "watchdog_thresh"
-(compile-time initialized to 10 and configurable through sysctl of the
-same name) seconds to check for hardlockups. If any CPU in the system
-does not receive any hrtimer interrupt during that time the
-'hardlockup detector' (the handler for the NMI perf event) will
-generate a kernel warning or call panic, depending on the
-configuration.
-
-The watchdog job runs in a stop scheduling thread that updates a
-timestamp every time it is scheduled. If that timestamp is not updated
-for 2*watchdog_thresh seconds (the softlockup threshold) the
+The soft and hard lockup detectors are built around an hrtimer.
+In addition, the softlockup detector regularly schedules a job, and
+the hard lockup detector might use Perf/NMI events on architectures
+that support it.
+
+Frequency and Heartbeats
+------------------------
+
+The core of the detectors is an hrtimer. It serves multiple purposes:
+
+- schedules watchdog job for the softlockup detector
+- bumps the interrupt counter for hardlockup detectors (heartbeat)
+- detects softlockups
+- detects hardlockups in Buddy mode
+
+The period of this hrtimer is 2*watchdog_thresh/5, which is 4 seconds
+by default. The hrtimer has two or three chances to generate an interrupt
+(heartbeat) before the hardlockup detector kicks in.
+
+Softlockup Detector
+-------------------
+
+The watchdog job is scheduled by the hrtimer and runs in a stop scheduling
+thread. It updates a timestamp every time it is scheduled. If that timestamp
+is not updated for 2*watchdog_thresh seconds (the softlockup threshold) the
 'softlockup detector' (coded inside the hrtimer callback function)
 will dump useful debug information to the system log, after which it
 will call panic if it was instructed to do so or resume execution of
 other kernel code.
 
-The period of the hrtimer is 2*watchdog_thresh/5, which means it has
-two or three chances to generate an interrupt before the hardlockup
-detector kicks in.
+Hardlockup Detector (NMI/Perf)
+------------------------------
+
+On architectures that support NMI (Non-Maskable Interrupt) perf events,
+a periodic NMI is generated every "watchdog_thresh" seconds.
+
+If any CPU in the system does not receive any hrtimer interrupt
+(heartbeat) during the "watchdog_thresh" window, the 'hardlockup
+detector' (the handler for the NMI perf event) will generate a kernel
+warning or call panic.
+
+**Detection Overhead (NMI):**
+
+The time to detect a lockup can vary depending on when the lockup
+occurs relative to the NMI check window. Examples below assume a watchdog_thresh of 10.
+
+* **Best Case:** The lockup occurs just before the first heartbeat is
+  due. The detector will notice the missing hrtimer interrupt almost
+  immediately during the next check.
+
+  ::
+
+    Time 100.0: cpu 1 heartbeat
+    Time 100.1: hardlockup_check, cpu1 stores its state
+    Time 103.9: Hard Lockup on cpu1
+    Time 104.0: cpu 1 heartbeat never comes
+    Time 110.1: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
+
+    Time to detection: ~6 seconds
+
+* **Worst Case:** The lockup occurs shortly after a valid interrupt
+  (heartbeat) which itself happened just after the NMI check. The next
+  NMI check sees that the interrupt count has changed (due to that one
+  heartbeat), assumes the CPU is healthy, and resets the baseline. The
+  lockup is only detected at the subsequent check.
+
+  ::
+
+    Time 100.0: hardlockup_check, cpu1 stores its state
+    Time 100.1: cpu 1 heartbeat
+    Time 100.2: Hard Lockup on cpu1
+    Time 110.0: hardlockup_check, cpu1 stores its state (misses lockup as state changed)
+    Time 120.0: hardlockup_check, cpu1 checks the state again, should be the same, declares lockup
+
+    Time to detection: ~20 seconds
+
+Hardlockup Detector (Buddy)
+---------------------------
+
+On architectures or configurations where NMI perf events are not
+available (or disabled), the kernel may use the "buddy" hardlockup
+detector. This mechanism requires SMP (Symmetric Multi-Processing).
+
+In this mode, each CPU is assigned a "buddy" CPU to monitor. The
+monitoring CPU runs its own hrtimer (the same one used for softlockup
+detection) and checks if the buddy CPU's hrtimer interrupt count has
+increased.
+
+To ensure timeliness and avoid false positives, the buddy system performs
+checks at every hrtimer interval (2*watchdog_thresh/5, which is 4 seconds
+by default). It uses a missed-interrupt threshold of 3. If the buddy's
+interrupt count has not changed for 3 consecutive checks, it is assumed
+that the buddy CPU is hardlocked (interrupts disabled). The monitoring
+CPU will then trigger the hardlockup response (warning or panic).
+
+**Detection Overhead (Buddy):**
+
+With a default check interval of 4 seconds (watchdog_thresh = 10):
+
+* **Best case:** Lockup occurs just before a check.
+    Detected in ~8s (0s till 1st check + 4s till 2nd + 4s till 3rd).
+* **Worst case:** Lockup occurs just after a check.
+    Detected in ~12s (4s till 1st check + 4s till 2nd + 4s till 3rd).
+
+**Limitations of the Buddy Detector:**
+
+1.  **All-CPU Lockup:** If all CPUs lock up simultaneously, the buddy
+    detector cannot detect the condition because the monitoring CPUs
+    are also frozen.
+2.  **Stack Traces:** Unlike the NMI detector, the buddy detector
+    cannot directly interrupt the locked CPU to grab a stack trace.
+    It relies on architecture-specific mechanisms (like NMI backtrace
+    support) to try and retrieve the status of the locked CPU. If
+    such support is missing, the log may only show that a lockup
+    occurred without providing the locked CPU's stack.
 
-As explained above, a kernel knob is provided that allows
-administrators to configure the period of the hrtimer and the perf
-event. The right value for a particular environment is a trade-off
-between fast response to lockups and detection overhead.
+Watchdog Core Exclusion
+=======================
 
 By default, the watchdog runs on all online cores.  However, on a
 kernel configured with NO_HZ_FULL, by default the watchdog runs only
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 9aed74e65cf4..c6994e55d141 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -418,7 +418,8 @@ hung_task_detect_count
 ======================
 
 Indicates the total number of tasks that have been detected as hung since
-the system boot.
+the system boot or since the counter was reset. The counter is zeroed when
+a value of 0 is written.
 
 This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.