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

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Date:   Wed, 19 Aug 2020 14:54:25 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Baoquan He <bhe@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Oscar Salvador <osalvador@...e.de>
Subject: 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:
> > 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@...ux-foundation.org>
> >> Cc: Michal Hocko <mhocko@...e.com>
> >> Cc: Wei Yang <richard.weiyang@...ux.alibaba.com>
> >> Cc: Baoquan He <bhe@...hat.com>
> >> Cc: Pankaj Gupta <pankaj.gupta.linux@...il.com>
> >> Cc: Oscar Salvador <osalvador@...e.de>
> >> Signed-off-by: David Hildenbrand <david@...hat.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).

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@...e.com>
-- 
Michal Hocko
SUSE Labs