linux-kernel - [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline

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Date:   Tue, 11 Jul 2023 19:40:50 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-doc@...r.kernel.org,
        David Hildenbrand <david@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Oscar Salvador <osalvador@...e.de>,
        Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v1] mm/memory_hotplug: document the signal_pending() check in offline_pages()

Let's update the documentation that any signal is sufficient, and
add a comment that not only checking for fatal signals is historical
baggage: changing it now could break existing user space. although
unlikely.

For example, when an app provides a custom SIGALRM handler and triggers
memory offlining, the timeout cmd would no longer stop memory offlining,
because SIGALRM would no longer be considered a fatal signal.

Cc: Michal Hocko <mhocko@...e.com>
Cc: Oscar Salvador <osalvador@...e.de>
Cc: Jonathan Corbet <corbet@....net>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: David Hildenbrand <david@...hat.com>
---
 Documentation/admin-guide/mm/memory-hotplug.rst | 2 +-
 mm/memory_hotplug.c                             | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst
index 1b02fe5807cc..bd77841041af 100644
--- a/Documentation/admin-guide/mm/memory-hotplug.rst
+++ b/Documentation/admin-guide/mm/memory-hotplug.rst
@@ -669,7 +669,7 @@ when still encountering permanently unmovable pages within ZONE_MOVABLE
 (-> BUG), memory offlining will keep retrying until it eventually succeeds.
 
 When offlining is triggered from user space, the offlining context can be
-terminated by sending a fatal signal. A timeout based offlining can easily be
+terminated by sending a signal. A timeout based offlining can easily be
 implemented via::
 
 	% timeout $TIMEOUT offline_block | failure_handling
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 3f231cf1b410..7cfd13c91568 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1843,6 +1843,11 @@ int __ref offline_pages(unsigned long start_pfn, unsigned long nr_pages,
 	do {
 		pfn = start_pfn;
 		do {
+			/*
+			 * Historically we always checked for any signal and
+			 * can't limit it to fatal signals without eventually
+			 * breaking user space.
+			 */
 			if (signal_pending(current)) {
 				ret = -EINTR;
 				reason = "signal backoff";
-- 
2.41.0