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

[Ralph Campbell]

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?).

> 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.