Messages in this thread | | | Subject | Re: [PATCH v1 02/11] mm/memory_hotplug: enforce section granularity when onlining/offlining | From | David Hildenbrand <> | Date | Wed, 19 Aug 2020 14:43:28 +0200 |
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On 19.08.20 14:37, Michal Hocko wrote: > On Wed 19-08-20 12:11:48, David Hildenbrand wrote: >> 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. > >> Cc: Andrew Morton <akpm@linux-foundation.org> >> Cc: Michal Hocko <mhocko@suse.com> >> Cc: Wei Yang <richard.weiyang@linux.alibaba.com> >> Cc: Baoquan He <bhe@redhat.com> >> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com> >> Cc: Oscar Salvador <osalvador@suse.de> >> Signed-off-by: David Hildenbrand <david@redhat.com> >> --- >> 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).
It's essentially a compressed version of
!nr_pages || !IS_ALIGNED(pfn, PAGES_PER_SECTION) || !IS_ALIGN(nr_pages, PAGES_PER_SECTION)
which is the same as
!nr_pages || pfn % PAGES_PER_SECTION) || nr_pages % PAGES_PER_SECTION
or
!nr_pages || (pfn | nr_pages) % PAGES_PER_SECTION
I consider IS_ALIGNED easier to read than % PAGES_PER_SECTION. I can certainly un-compress, whatever you prefer, thanks.
-- Thanks,
David / dhildenb
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