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

From: Ralph Campbell
Date: Mon Jun 22 2020 - 17:32:00 EST


On 6/22/20 1:10 PM, Zi Yan wrote:
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@xxxxxxxxxx>
---
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'll fix this up for v2 and you can double check me.

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

OK.

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.

I have compiled and run with CONFIG_DEVICE_PRIVATE off but I can test more combinations of
config settings.


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.

Sounds reasonable. I'll work on adding the fallback path for v2.