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Message-ID: <C7BEB563-3698-442C-A188-1B66CBE4CF63@nvidia.com>
Date: Mon, 22 Jun 2020 16:10:45 -0400
From: Zi Yan <ziy@...dia.com>
To: Ralph Campbell <rcampbell@...dia.com>
CC: <nouveau@...ts.freedesktop.org>, <linux-rdma@...r.kernel.org>,
<linux-mm@...ck.org>, <linux-kselftest@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, Jerome Glisse <jglisse@...hat.com>,
"John Hubbard" <jhubbard@...dia.com>,
Christoph Hellwig <hch@....de>,
"Jason Gunthorpe" <jgg@...lanox.com>,
Ben Skeggs <bskeggs@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 13/16] mm: support THP migration to device private memory
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@...dia.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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