[PATCH v5 3/4] Documentation: document panic_on_unrecoverable_memory_failure sysctl

[Breno Leitao <leitao@debian.org>]

Add documentation for the new vm.panic_on_unrecoverable_memory_failure
sysctl, describing the three categories of failures that trigger a
panic and noting which kernel page types are not yet covered.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 Documentation/admin-guide/sysctl/vm.rst | 65 +++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
index 97e12359775c9..f118ec5cd1fad 100644
--- a/Documentation/admin-guide/sysctl/vm.rst
+++ b/Documentation/admin-guide/sysctl/vm.rst
@@ -67,6 +67,7 @@ Currently, these files are in /proc/sys/vm:
 - page-cluster
 - page_lock_unfairness
 - panic_on_oom
+- panic_on_unrecoverable_memory_failure
 - percpu_pagelist_high_fraction
 - stat_interval
 - stat_refresh
@@ -925,6 +926,70 @@ panic_on_oom=2+kdump gives you very strong tool to investigate
 why oom happens. You can get snapshot.
 
 
+panic_on_unrecoverable_memory_failure
+======================================
+
+When a hardware memory error (e.g. multi-bit ECC) hits a kernel page
+that cannot be recovered by the memory failure handler, the default
+behaviour is to ignore the error and continue operation.  This is
+dangerous because the corrupted data remains accessible to the kernel,
+risking silent data corruption or a delayed crash when the poisoned
+memory is next accessed.
+
+When enabled, this sysctl triggers a panic on three categories of
+unrecoverable failures: reserved kernel pages, non-buddy kernel pages
+with zero refcount (e.g. tail pages of high-order allocations), and
+pages whose state cannot be classified as recoverable.
+
+Note that some kernel page types — such as slab objects, vmalloc
+allocations, kernel stacks, and page tables — share a failure path
+with transient refcount races and are not currently covered by this
+option. I.e, do not panic when not confident of the page status.
+
+For many environments it is preferable to panic immediately with a clean
+crash dump that captures the original error context, rather than to
+continue and face a random crash later whose cause is difficult to
+diagnose.
+
+Use cases
+---------
+
+This option is most useful in environments where unattributed crashes
+are expensive to debug or where data integrity must take precedence
+over availability:
+
+* Large fleets, where multi-bit ECC errors on kernel pages are observed
+  regularly and post-mortem analysis of an unrelated downstream crash
+  (often seconds to minutes after the original error) consumes
+  significant engineering effort.
+
+* Systems configured with kdump, where panicking at the moment of the
+  hardware error produces a vmcore that still contains the faulting
+  address, the affected page state, and the originating MCE/GHES
+  record — context that is typically lost by the time a delayed crash
+  occurs.
+
+* High-availability clusters that rely on fast, deterministic node
+  failure for failover, and prefer an immediate panic over silent data
+  corruption propagating to replicas or persistent storage.
+
+* Kernel and platform developers reproducing hwpoison issues with
+  tools such as ``mce-inject`` or error-injection debugfs interfaces,
+  where panicking on the unrecoverable path makes regressions
+  immediately visible instead of surfacing as later, unrelated
+  failures.
+
+= =====================================================================
+0 Try to continue operation (default).
+1 Panic immediately.  If the ``panic`` sysctl is also non-zero then the
+  machine will be rebooted.
+= =====================================================================
+
+Example::
+
+     echo 1 > /proc/sys/vm/panic_on_unrecoverable_memory_failure
+
+
 percpu_pagelist_high_fraction
 =============================
 

-- 
2.52.0