Re: [PATCH 13/16] mm: support THP migration to device private memory

[Zi Yan <ziy@nvidia.com>]

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On 22 Jun 2020, at 15:36, Ralph Campbell wrote:

> On 6/21/20 4:20 PM, Zi Yan wrote:
>> On 19 Jun 2020, at 17:56, Ralph Campbell wrote:
>>
>>> Support transparent huge page migration to ZONE_DEVICE private memory.
>>> A new flag (MIGRATE_PFN_COMPOUND) is added to the input PFN array to
>>> indicate the huge page was fully mapped by the CPU.
>>> Export prep_compound_page() so that device drivers can create huge
>>> device private pages after calling memremap_pages().
>>>
>>> Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
>>> ---
>>>   include/linux/migrate.h |   1 +
>>>   include/linux/mm.h      |   1 +
>>>   mm/huge_memory.c        |  30 ++++--
>>>   mm/internal.h           |   1 -
>>>   mm/memory.c             |  10 +-
>>>   mm/memremap.c           |   9 +-
>>>   mm/migrate.c            | 226 ++++++++++++++++++++++++++++++++--------
>>>   mm/page_alloc.c         |   1 +
>>>   8 files changed, 226 insertions(+), 53 deletions(-)
>>>
>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
>>> index 3e546cbf03dd..f6a64965c8bd 100644
>>> --- a/include/linux/migrate.h
>>> +++ b/include/linux/migrate.h
>>> @@ -166,6 +166,7 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,
>>>   #define MIGRATE_PFN_MIGRATE	(1UL << 1)
>>>   #define MIGRATE_PFN_LOCKED	(1UL << 2)
>>>   #define MIGRATE_PFN_WRITE	(1UL << 3)
>>> +#define MIGRATE_PFN_COMPOUND	(1UL << 4)
>>>   #define MIGRATE_PFN_SHIFT	6
>>>
>>>   static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index dc7b87310c10..020b9dd3cddb 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -932,6 +932,7 @@ static inline unsigned int page_shift(struct page *page)
>>>   }
>>>
>>>   void free_compound_page(struct page *page);
>>> +void prep_compound_page(struct page *page, unsigned int order);
>>>
>>>   #ifdef CONFIG_MMU
>>>   /*
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 78c84bee7e29..25d95f7b1e98 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -1663,23 +1663,35 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>>   	} else {
>>>   		struct page *page = NULL;
>>>   		int flush_needed = 1;
>>> +		bool is_anon = false;
>>>
>>>   		if (pmd_present(orig_pmd)) {
>>>   			page = pmd_page(orig_pmd);
>>> +			is_anon = PageAnon(page);
>>>   			page_remove_rmap(page, true);
>>>   			VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
>>>   			VM_BUG_ON_PAGE(!PageHead(page), page);
>>>   		} else if (thp_migration_supported()) {
>>>   			swp_entry_t entry;
>>>
>>> -			VM_BUG_ON(!is_pmd_migration_entry(orig_pmd));
>>>   			entry = pmd_to_swp_entry(orig_pmd);
>>> -			page = pfn_to_page(swp_offset(entry));
>>> +			if (is_device_private_entry(entry)) {
>>> +				page = device_private_entry_to_page(entry);
>>> +				is_anon = PageAnon(page);
>>> +				page_remove_rmap(page, true);
>>> +				VM_BUG_ON_PAGE(page_mapcount(page) < 0, page);
>>> +				VM_BUG_ON_PAGE(!PageHead(page), page);
>>> +				put_page(page);
>>
>> Why do you hide this code behind thp_migration_supported()? It seems that you just need
>> pmd swap entry not pmd migration entry. Also the condition is not consistent with the code
>> in __handle_mm_fault(), in which you handle is_device_private_entry() directly without
>> checking thp_migration_support().
>
> Good point, I think "else if (thp_migration_supported())" should be
> "else if (is_pmd_migration_entry(orig_pmd))" since if the PMD *is*
> a device private or migration entry, then it should be handled and the
> VM_BUG_ON() should be that thp_migration_supported() is true
> (or maybe remove the VM_BUG_ON?).

I disagree. A device private entry is independent of a PMD migration entry, since a device private
entry is just a swap entry, which is available when CONFIG_TRANSPARENT_HUGEPAGE. So for architectures
support THP but not THP migration (like ARM64), your code should still work.

I would suggest you to check all the use of is_swap_pmd() and make sure the code
can handle is_device_private_entry().

For new device private code, you might need to guard it either statically or dynamically in case
CONFIG_DEVICE_PRIVATE is disabled. Potentially, you would like to make sure a system without
CONFIG_DEVICE_PRIVATE will not see is_device_private_entry() == true and give errors when it does.


>
>> Do we need to support split_huge_pmd() if a page is migrated to device? Any new code
>> needed in split_huge_pmd()?
>
> I was thinking that any CPU usage of the device private page would cause it to be
> migrated back to system memory as a whole PMD/PUD page but I'll double check.
> At least there should be a check that the page isn't a device private page.

Well, that depends. If we can allocate a THP on CPU memory, we can migrate the whole page back.
But if no THP is allocated due to low on free memory or memory fragmentation, I think you
might need a fallback plan, either splitting the device private page and migrating smaller
pages instead or reclaiming CPU memory until you get a THP. IMHO, the former might be preferred,
since the latter might cost a lot of CPU cycles but still gives no THP after all.



—
Best Regards,
Yan Zi

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