Re: [PATCH v2 1/2] arm64: mm: vmemmap populate to page level if not section aligned

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

Thanks Catalin for review!
Merry Christmas.

On 2024/12/21 2:30, Catalin Marinas wrote:
> On Mon, Dec 09, 2024 at 05:42:26PM +0800, Zhenhua Huang wrote:
>> Commit c1cc1552616d ("arm64: MMU initialisation")
>> optimizes the vmemmap to populate at the PMD section level. However, if
>> start or end is not aligned to a section boundary, such as when a
>> subsection is hot added, populating the entire section is wasteful. For
>> instance, if only one subsection hot-added, the entire section's struct
>> page metadata will still be populated.In such cases, it is more effective
>> to populate at page granularity.
> 
> OK, so from the vmemmap perspective, we waste up to 2MB memory that has
> been allocated even if a 2MB hot-plugged subsection required only 32KB
> of struct page. I don't mind this much really. I hope all those
> subsections are not scattered around to amplify this waste.
> 
>> This change also addresses mismatch issues during vmemmap_free(): When
>> pmd_sect() is true, the entire PMD section is cleared, even if there is
>> other effective subsection. For example, pagemap1 and pagemap2 are part
>> of a single PMD entry and they are hot-added sequentially. Then pagemap1
>> is removed, vmemmap_free() will clear the entire PMD entry, freeing the
>> struct page metadata for the whole section, even though pagemap2 is still
>> active.
> 
> I think that's the bigger issue. We can't unplug a subsection only.
> Looking at unmap_hotplug_pmd_range(), it frees a 2MB vmemmap section but
> that may hold struct page for the equivalent of 128MB of memory. So any
> struct page accesses for the other subsections will fault.

Exactly! That's what the patch aims to address.

> 
>> Fixes: c1cc1552616d ("arm64: MMU initialisation")
> 
> I wouldn't add a fix for the first commit adding arm64 support, we did
> not even have memory hotplug at the time (added later in 5.7 by commit
> bbd6ec605c0f ("arm64/mm: Enable memory hot remove")). IIUC, this hasn't
> been a problem until commit ba72b4c8cf60 ("mm/sparsemem: support
> sub-section hotplug"). That commit broke some arm64 assumptions.

Shall we add ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug") 
because it broke arm64 assumptions ?

> 
>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
>> ---
>>   arch/arm64/mm/mmu.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index e2739b69e11b..fd59ee44960e 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1177,7 +1177,9 @@ int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
>>   {
>>   	WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
>>   
>> -	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
>> +	if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
>> +	!IS_ALIGNED(page_to_pfn((struct page *)start), PAGES_PER_SECTION) ||
>> +	!IS_ALIGNED(page_to_pfn((struct page *)end), PAGES_PER_SECTION))
>>   		return vmemmap_populate_basepages(start, end, node, altmap);
>>   	else
>>   		return vmemmap_populate_hugepages(start, end, node, altmap);
> 
> An alternative would be to fix unmap_hotplug_pmd_range() etc. to avoid
> nuking the whole vmemmap pmd section if it's not empty. Not sure how
> easy that is, whether we have the necessary information (I haven't
> looked in detail).
> 
> A potential issue - can we hotplug 128MB of RAM and only unplug 2MB? If
> that's possible, the problem isn't solved by this patch.

Indeed, seems there is no guarantee that plug size must be equal to 
unplug size...

I have two ideas:
1. Completely disable this PMD mapping optimization since there is no 
guarantee we must align 128M memory for hotplug ..

2. If we want to take this optimization.
I propose adding one argument to vmemmap_free to indicate if the entire 
section is freed(based on subsection map). Vmemmap_free is a common 
function and might affect other architectures... The process would be:
vmemmap_free
	unmap_hotplug_range //In unmap_hotplug_pmd_range() as you mentioned：if 
whole section is freed, proceed as usual. Otherwise, *just clear out 
struct page content but do not free*.
	free_empty_tables // will be called only if entire section is freed

On the populate side,
else if (vmemmap_check_pmd(pmd, node, addr, next)) //implement this function
	continue;	//Buffer still exists, just abort..

Could you please comment further whether #2 is feasible ?

>