Re: [PATCH v1 02/11] mm/memory_hotplug: enforce section granularity when onlining/offlining

On Wed 19-08-20 14:43:28, David Hildenbrand wrote:

On 19.08.20 14:37, Michal Hocko wrote:

quoted

On Wed 19-08-20 12:11:48, David Hildenbrand wrote:

quoted

Already two people (including me) tried to offline subsections, because
the function looks like it can deal with it. But we really can only
online/offline full sections (e.g., we can only mark full sections
online/offline via SECTION_IS_ONLINE).

Add a simple safety net that to document the restriction now. Current users
(core and powernv/memtrace) respect these restrictions.

I do agree with the warning because it clarifies our expectations
indeed. Se below for more questions.

quoted

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Wei Yang <redacted>
Cc: Baoquan He <redacted>
Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: David Hildenbrand <redacted>
---
 mm/memory_hotplug.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index c781d386d87f9..6856702af68d9 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c

@@ -801,6 +801,11 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
 	int ret;
 	struct memory_notify arg;
 
+	/* We can only online full sections (e.g., SECTION_IS_ONLINE) */
+	if (WARN_ON_ONCE(!nr_pages ||
+			 !IS_ALIGNED(pfn | nr_pages, PAGES_PER_SECTION)))
+		return -EINVAL;

This looks looks unnecessarily cryptic to me. Do you want to catch full
section operation that doesn't start at the usual section boundary? If
yes the above doesn't work work unless I am missing something.

Why don't you simply WARN_ON_ONCE(nr_pages % PAGES_PER_SECTION).
!nr_pages doesn't sound like something interesting to care about or why
do we care?

Also the start pfn has to be section aligned, so we always cover fully
aligned sections (e.g., not two partial ones).

OK, I've misread your intention. I thought that we check for the start
pfn prior to this warning but we only do that after.

Acked-by: Michal Hocko <mhocko@suse.com>
-- 
Michal Hocko
SUSE Labs