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Message-ID: <1f0ef337-6ca5-54fa-e627-41a46be73f2b@nvidia.com>
Date: Mon, 29 Jul 2019 16:42:01 -0700
From: Ralph Campbell <rcampbell@...dia.com>
To: Christoph Hellwig <hch@....de>,
Jérôme Glisse <jglisse@...hat.com>,
Jason Gunthorpe <jgg@...lanox.com>,
Ben Skeggs <bskeggs@...hat.com>
CC: Bharata B Rao <bharata@...ux.ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>,
<linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
<dri-devel@...ts.freedesktop.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 9/9] mm: remove the MIGRATE_PFN_WRITE flag
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@....de>
Reviewed-by: Ralph Campbell <rcampbell@...dia.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.
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