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Subject: Re: [PATCH v1 2/5] mm/memory_hotplug: enforce section alignment when
 onlining/offlining
To:     Pasha Tatashin <Pavel.Tatashin@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Cc:     "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Kemi Wang <kemi.wang@intel.com>,
        David Rientjes <rientjes@google.com>,
        Jia He <jia.he@hxt-semitech.com>,
        Oscar Salvador <osalvador@suse.de>,
        Petr Tesarik <ptesarik@suse.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Mathieu Malaterre <malat@debian.org>,
        Baoquan He <bhe@redhat.com>,
        Wei Yang <richard.weiyang@gmail.com>,
        Ross Zwisler <zwisler@kernel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
References: <20180816100628.26428-1-david@redhat.com>
 <20180816100628.26428-3-david@redhat.com>
 <772774b8-77d8-c09b-f933-5ce29be58fa9@microsoft.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <bd09cf13-6eba-ed06-5491-0d1e239c40c8@redhat.com>
Date:   Fri, 31 Aug 2018 09:51:07 +0200
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On 31.08.2018 00:14, Pasha Tatashin wrote:
> Hi David,
> 
> I am not sure this is needed, because we already have a stricter checker:
> 
> check_hotplug_memory_range()
> 
> You could call it from online_pages(), if you think there is a reason to
> do it, but other than that it is done from add_memory_resource() and
> from remove_memory().

Hi,

As offline_pages() is called from a different location for ppc (and I
understand why but don't consider this clean) and I used both functions
in a prototype, believing they would work with pageblock_nr_pages, I
really think that we should at least drop the misleading check from
offline_pages() and better also add checks for check_hotplug_memory_range().

Thanks for having a look Pavel!

> 
> Thank you,
> Pavel
> 
> On 8/16/18 6:06 AM, David Hildenbrand wrote:
>> onlining/offlining code works on whole sections, so let's enforce that.
>> Existing code only allows to add memory in memory block size. And only
>> whole memory blocks can be onlined/offlined. Memory blocks are always
>> aligned to sections, so this should not break anything.
>>
>> online_pages/offline_pages will implicitly mark whole sections
>> online/offline, so the code really can only handle such granularities.
>>
>> (especially offlining code cannot deal with pageblock_nr_pages but
>>  theoretically only MAX_ORDER-1)
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>  mm/memory_hotplug.c | 10 +++++++---
>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>> index 090cf474de87..30d2fa42b0bb 100644
>> --- a/mm/memory_hotplug.c
>> +++ b/mm/memory_hotplug.c
>> @@ -897,6 +897,11 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages, int online_typ
>>  	struct memory_notify arg;
>>  	struct memory_block *mem;
>>  
>> +	if (!IS_ALIGNED(pfn, PAGES_PER_SECTION))
>> +		return -EINVAL;
>> +	if (!IS_ALIGNED(nr_pages, PAGES_PER_SECTION))
>> +		return -EINVAL;
>> +
>>  	/*
>>  	 * We can't use pfn_to_nid() because nid might be stored in struct page
>>  	 * which is not yet initialized. Instead, we find nid from memory block.
>> @@ -1600,10 +1605,9 @@ int offline_pages(unsigned long start_pfn, unsigned long nr_pages)
>>  	struct zone *zone;
>>  	struct memory_notify arg;
>>  
>> -	/* at least, alignment against pageblock is necessary */
>> -	if (!IS_ALIGNED(start_pfn, pageblock_nr_pages))
>> +	if (!IS_ALIGNED(start_pfn, PAGES_PER_SECTION))
>>  		return -EINVAL;
>> -	if (!IS_ALIGNED(end_pfn, pageblock_nr_pages))
>> +	if (!IS_ALIGNED(nr_pages, PAGES_PER_SECTION))
>>  		return -EINVAL;
>>  	/* This makes hotplug much easier...and readable.
>>  	   we assume this for now. .*/


-- 

Thanks,

David / dhildenb