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