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Subject: Re: [PATCH 9/9] mm: remove the MIGRATE_PFN_WRITE flag
To:     Christoph Hellwig <hch@lst.de>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Ben Skeggs <bskeggs@redhat.com>
CC:     Bharata B Rao <bharata@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-mm@kvack.org>, <nouveau@lists.freedesktop.org>,
        <dri-devel@lists.freedesktop.org>, <linux-kernel@vger.kernel.org>
References: <20190729142843.22320-1-hch@lst.de>
 <20190729142843.22320-10-hch@lst.de>
From:   Ralph Campbell <rcampbell@nvidia.com>
Message-ID: <1f0ef337-6ca5-54fa-e627-41a46be73f2b@nvidia.com>
Date:   Mon, 29 Jul 2019 16:42:01 -0700
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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.