linux-ext4 - Re: [PATCH v5 02/10] mm: add device coherent vma selection for memory migration

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Date:   Mon, 31 Jan 2022 17:32:09 +1100
From:   Alistair Popple <apopple@...dia.com>
To:     <akpm@...ux-foundation.org>, <Felix.Kuehling@....com>,
        <linux-mm@...ck.org>, <rcampbell@...dia.com>,
        <linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
        Alex Sierra <alex.sierra@....com>
CC:     <amd-gfx@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
        <hch@....de>, <jgg@...dia.com>, <jglisse@...hat.com>,
        <willy@...radead.org>
Subject: Re: [PATCH v5 02/10] mm: add device coherent vma selection for memory migration

On Saturday, 29 January 2022 7:08:17 AM AEDT Alex Sierra wrote:

[...]

>  struct migrate_vma {
> diff --git a/mm/migrate.c b/mm/migrate.c
> index cd137aedcfe5..d3cc3589e1e8 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2264,7 +2264,8 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  			if (is_writable_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_DEVICE_COHERENT))
>  				goto next;
>  			pfn = pte_pfn(pte);
>  			if (is_zero_pfn(pfn)) {

Sorry, but I still don't think this is quite right.

When specifying MIGRATE_VMA_SELECT_DEVICE_COHERENT we are looking for pages to
migrate from the device back to system memory. But as currently written I think
this can also select the zero pfn when MIGRATE_VMA_SELECT_DEVICE_COHERENT is
specified. As far as I know that can never point to device memory so migration
of a zero pfn should be also be skipped in that case.

We should only migrate the zero pfn if MIGRATE_VMA_SELECT_SYSTEM is specified.

> @@ -2273,6 +2274,13 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>  				goto next;
>  			}
>  			page = vm_normal_page(migrate->vma, addr, pte);
> +			if (page && !is_zone_device_page(page) &&
> +			    !(migrate->flags & MIGRATE_VMA_SELECT_SYSTEM))
> +				goto next;
> +			if (page && is_device_coherent_page(page) &&
> +			    (!(migrate->flags & MIGRATE_VMA_SELECT_DEVICE_COHERENT) ||
> +			     page->pgmap->owner != migrate->pgmap_owner))
> +				goto next;
>  			mpfn = migrate_pfn(pfn) | MIGRATE_PFN_MIGRATE;
>  			mpfn |= pte_write(pte) ? MIGRATE_PFN_WRITE : 0;
>  		}
>