Re: [PATCH 9/9] mm: remove the MIGRATE_PFN_WRITE flag

[Jerome Glisse <jglisse@redhat.com>]

On Mon, Jul 29, 2019 at 04:42:01PM -0700, Ralph Campbell wrote:
> 
> On 7/29/19 7:28 AM, Christoph Hellwig wrote:
> > The MIGRATE_PFN_WRITE is only used locally in migrate_vma_collect_pmd,
> > where it can be replaced with a simple boolean local variable.
> > 
> > Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
> 
> > ---
> >   include/linux/migrate.h | 1 -
> >   mm/migrate.c            | 9 +++++----
> >   2 files changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> > index 8b46cfdb1a0e..ba74ef5a7702 100644
> > --- a/include/linux/migrate.h
> > +++ b/include/linux/migrate.h
> > @@ -165,7 +165,6 @@ static inline int migrate_misplaced_transhuge_page(struct mm_struct *mm,
> >   #define MIGRATE_PFN_VALID	(1UL << 0)
> >   #define MIGRATE_PFN_MIGRATE	(1UL << 1)
> >   #define MIGRATE_PFN_LOCKED	(1UL << 2)
> > -#define MIGRATE_PFN_WRITE	(1UL << 3)
> >   #define MIGRATE_PFN_SHIFT	6
> >   static inline struct page *migrate_pfn_to_page(unsigned long mpfn)
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 74735256e260..724f92dcc31b 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -2212,6 +2212,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >   		unsigned long mpfn, pfn;
> >   		struct page *page;
> >   		swp_entry_t entry;
> > +		bool writable = false;
> >   		pte_t pte;
> >   		pte = *ptep;
> > @@ -2240,7 +2241,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >   			mpfn = migrate_pfn(page_to_pfn(page)) |
> >   					MIGRATE_PFN_MIGRATE;
> >   			if (is_write_device_private_entry(entry))
> > -				mpfn |= MIGRATE_PFN_WRITE;
> > +				writable = true;
> >   		} else {
> >   			if (is_zero_pfn(pfn)) {
> >   				mpfn = MIGRATE_PFN_MIGRATE;
> > @@ -2250,7 +2251,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >   			}
> >   			page = vm_normal_page(migrate->vma, addr, pte);
> >   			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
> > -			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
> > +			if (pte_write(pte))
> > +				writable = true;
> >   		}
> >   		/* FIXME support THP */
> > @@ -2284,8 +2286,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >   			ptep_get_and_clear(mm, addr, ptep);
> >   			/* Setup special migration page table entry */
> > -			entry = make_migration_entry(page, mpfn &
> > -						     MIGRATE_PFN_WRITE);
> > +			entry = make_migration_entry(page, writable);
> >   			swp_pte = swp_entry_to_pte(entry);
> >   			if (pte_soft_dirty(pte))
> >   				swp_pte = pte_swp_mksoft_dirty(swp_pte);
> > 
> 
> MIGRATE_PFN_WRITE may mot being used but that seems like a bug to me.
> If a page is migrated to device memory, it could be mapped at the same
> time to avoid a device page fault but it would need the flag to know
> whether to map it RW or RO. But I suppose that could be inferred from
> the vma->vm_flags.

It is a bug that it is not being use right now. I will have to dig my
git repo to see when that got kill. Will look into it once i get back.

The vma->vm_flags is of no use here. A page can be write protected
inside a writable vma for various reasons.

Cheers,
Jérôme