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

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Message-ID: <d9e82d2e-0786-ebfd-acc3-7dcc5ec6ad9b@redhat.com>
Date:   Wed, 19 Aug 2020 14:43:28 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...e.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 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).

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