Subject: Re: [PATCH V7 3/3] arm64/mm: Enable memory hot remove

From: Anshuman Khandual <[email-masked]>
Date: Thu, 12 Sep 2019 14:07:49 +0530

On 09/12/2019 09:58 AM, Anshuman Khandual wrote:
> 
> On 09/10/2019 09:47 PM, Catalin Marinas wrote:
>> On Tue, Sep 03, 2019 at 03:15:58PM +0530, Anshuman Khandual wrote:
>>> @@ -770,6 +1022,28 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>>  void vmemmap_free(unsigned long start, unsigned long end,
>>>  		struct vmem_altmap *altmap)
>>>  {
>>> +#ifdef CONFIG_MEMORY_HOTPLUG
>>> +	/*
>>> +	 * FIXME: We should have called remove_pagetable(start, end, true).
>>> +	 * vmemmap and vmalloc virtual range might share intermediate kernel
>>> +	 * page table entries. Removing vmemmap range page table pages here
>>> +	 * can potentially conflict with a concurrent vmalloc() allocation.
>>> +	 *
>>> +	 * This is primarily because vmalloc() does not take init_mm ptl for
>>> +	 * the entire page table walk and it's modification. Instead it just
>>> +	 * takes the lock while allocating and installing page table pages
>>> +	 * via [p4d|pud|pmd|pte]_alloc(). A concurrently vanishing page table
>>> +	 * entry via memory hot remove can cause vmalloc() kernel page table
>>> +	 * walk pointers to be invalid on the fly which can cause corruption
>>> +	 * or worst, a crash.
>>> +	 *
>>> +	 * So free_empty_tables() gets called where vmalloc and vmemmap range
>>> +	 * do not overlap at any intermediate level kernel page table entry.
>>> +	 */
>>> +	unmap_hotplug_range(start, end, true);
>>> +	if (!vmalloc_vmemmap_overlap)
>>> +		free_empty_tables(start, end);
>>> +#endif
>>>  }
>>>  #endif	/* CONFIG_SPARSEMEM_VMEMMAP */
> Hello Catalin,
> 
>> I wonder whether we could simply ignore the vmemmap freeing altogether,
>> just leave it around and not unmap it. This way, we could call
> This would have been an option (even if we just ignore for a moment that
> it might not be the cleanest possible method) if present memory hot remove
> scenarios involved just system RAM of comparable sizes.
> 
> But with persistent memory which will be plugged in as ZONE_DEVICE might
> ask for a vmem_atlamp based vmemmap mapping where the backing memory comes
> from the persistent memory range itself not from existing system RAM. IIRC
> altmap support was originally added because the amount persistent memory on
> a system might be order of magnitude higher than that of regular system RAM.
> During normal memory hot add (without altmap) would have caused great deal
> of consumption from system RAM just for persistent memory range's vmemmap
> mapping. In order to avoid such a scenario altmap was created to allocate
> vmemmap mapping backing memory from the device memory range itself.
> 
> In such cases vmemmap must be unmapped and it's backing memory freed up for
> the complete removal of persistent memory which originally requested for
> altmap based vmemmap backing.
> 
> Just as a reference, the upcoming series which enables altmap support on
> arm64 tries to allocate vmemmap mapping backing memory from the device range
> itself during memory hot add and free them up during memory hot remove. Those
> methods will not be possible if memory hot-remove does not really free up
> vmemmap backing storage.
> 
> https://patchwork.kernel.org/project/linux-mm/list/?series=139299
> 

Just to add in here. There is an ongoing work which will enable allocating
memory from the hot-add range itself even for normal system RAM. So this
might not be specific to ZONE_DEVICE based device/persistent memory alone
for a long time.

https://lore.kernel.org/lkml/[email-masked]/

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