linux-kernel - Re: [PATCH v2 4/7] mm,memory_hotplug: Enforce struct page size to be multiple of PMD

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Date:   Mon, 1 Mar 2021 11:24:57 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>, VlastimilBabkavbabka@...e.cz,
        pasha.tatashin@...een.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v2 4/7] mm,memory_hotplug: Enforce struct page size to be
 multiple of PMD

On 01.03.21 11:06, Oscar Salvador wrote:
> On Thu, Feb 25, 2021 at 07:26:50PM +0100, David Hildenbrand wrote:
>> On 09.02.21 14:38, Oscar Salvador wrote:
>>> When struct page's size is not multiple of PMD, these do not get
>>> fully populated when adding sections, hence two sections will
>>> intersect the same the PMD. This goes against the vmemmap-per-device
>>> premise, so reject it if that is the case.
>>>
>>> Signed-off-by: Oscar Salvador <osalvador@...e.de>
>>> ---
>>>    mm/memory_hotplug.c | 5 +++--
>>>    1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
>>> index 95695483a622..d3fb036d33fd 100644
>>> --- a/mm/memory_hotplug.c
>>> +++ b/mm/memory_hotplug.c
>>> @@ -57,10 +57,11 @@ static __meminit int memmap_on_memory_store(const char *val,
>>>    					    const struct kernel_param *kp)
>>>    {
>>>    	/*
>>> -	 * Fail silently in case we cannot enable it due to platform constraints.
>>> +	 * Fail silently in case we cannot enable it due to system constraints.
>>>    	 * User can always check whether it is enabled or not via /sys/module.
>>>    	 */
>>> -	if (!IS_ENABLED(CONFIG_ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE))
>>> +	if (!IS_ENABLED(CONFIG_ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE) ||
>>> +	    (PMD_SIZE % sizeof(struct page)))
>>>    		return 0;
>>>    	return param_set_bool(val, kp);
>>>
>>
>> Squash that into #1 - it's part of basic operation.
> 
> Just to be sure we are on the same page.
> You mean to move this patch and patch#5 into the runtime check in
> mhp_supports_memmap_on_memory(), right?

Yes, then the compiler might even be able to figure out that what 
follows the check is dead code.

-- 
Thanks,

David / dhildenb