linux-ext4 - Re: [PATCH v1 03/14] mm: add iomem vma selection for memory migration

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Date:   Wed, 25 Aug 2021 09:40:29 +0200
From:   Christoph Hellwig <hch@....de>
To:     Alex Sierra <alex.sierra@....com>
Cc:     akpm@...ux-foundation.org, Felix.Kuehling@....com,
        linux-mm@...ck.org, rcampbell@...dia.com,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        hch@....de, jgg@...dia.com, jglisse@...hat.com
Subject: Re: [PATCH v1 03/14] mm: add iomem vma selection for memory
 migration

On Tue, Aug 24, 2021 at 10:48:17PM -0500, Alex Sierra wrote:
> In this case, this is used to migrate pages from device memory, back to
> system memory. This particular device memory type should be accessible
> by the CPU, through IOMEM access. Typically, zone device public type
> memory falls into this category.
> 
> Signed-off-by: Alex Sierra <alex.sierra@....com>
> ---
>  include/linux/migrate.h | 1 +
>  mm/migrate.c            | 3 ++-
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/migrate.h b/include/linux/migrate.h
> index 4bb4e519e3f5..6b16f417384f 100644
> --- a/include/linux/migrate.h
> +++ b/include/linux/migrate.h
> @@ -156,6 +156,7 @@ static inline unsigned long migrate_pfn(unsigned long pfn)
>  enum migrate_vma_direction {
>  	MIGRATE_VMA_SELECT_SYSTEM = 1 << 0,
>  	MIGRATE_VMA_SELECT_DEVICE_PRIVATE = 1 << 1,
> +	MIGRATE_VMA_SELECT_IOMEM = 1 << 2,
>  };
>  
>  struct migrate_vma {
> diff --git a/mm/migrate.c b/mm/migrate.c
> index e3a10e2a1bb3..d4ae2da99607 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2406,7 +2406,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			if (is_write_device_private_entry(entry))
>  				mpfn |= MIGRATE_PFN_WRITE;
>  		} else {
> -			if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
> +			if (!(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM) &&
> +			    !(migrate->flags & MIGRATE_VMA_SELECT_IOMEM))

This makes the MIGRATE_VMA_SELECT_SYSTEM and MIGRATE_VMA_SELECT_IOMEM
behave entirely identifical, that is redundant.  I think we need to
distinguish between the dfferent cases here.  I think the right check
would be pfn_valid(), which should be true for system memory, and
false for iomem.

Also shouldn't this be called DEVICE_PUBLIC instead of IOMEM?