On Wed 19-08-20 14:43:28, David Hildenbrand wrote: > 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@xxxxxxxxxxxxxxxxxxxx> > >> Cc: Michal Hocko <mhocko@xxxxxxxx> > >> Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx> > >> Cc: Baoquan He <bhe@xxxxxxxxxx> > >> Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx> > >> Cc: Oscar Salvador <osalvador@xxxxxxx> > >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> > >> --- > >> 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@xxxxxxxx> -- Michal Hocko SUSE Labs
