linux-kernel - Re: [PATCH v7 5/8] mm: Device exclusive memory access

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Message-ID: <20210330193234.GA2356281@nvidia.com>
Date:   Tue, 30 Mar 2021 16:32:34 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, nouveau@...ts.freedesktop.org,
        bskeggs@...hat.com, akpm@...ux-foundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        jhubbard@...dia.com, rcampbell@...dia.com, jglisse@...hat.com,
        hch@...radead.org, daniel@...ll.ch, willy@...radead.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 5/8] mm: Device exclusive memory access

On Fri, Mar 26, 2021 at 11:08:02AM +1100, Alistair Popple wrote:
> diff --git a/mm/memory.c b/mm/memory.c
> index 3a5705cfc891..33d11527ef77 100644
> +++ b/mm/memory.c
> @@ -781,6 +781,27 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  				pte = pte_swp_mkuffd_wp(pte);
>  			set_pte_at(src_mm, addr, src_pte, pte);
>  		}
> +	} else if (is_device_exclusive_entry(entry)) {
> +		page = pfn_swap_entry_to_page(entry);
> +
> +		get_page(page);
> +		rss[mm_counter(page)]++;
> +
> +		if (is_writable_device_exclusive_entry(entry) &&
> +		    is_cow_mapping(vm_flags)) {
> +			/*
> +			 * COW mappings require pages in both
> +			 * parent and child to be set to read.
> +			 */
> +			entry = make_readable_device_exclusive_entry(
> +							swp_offset(entry));
> +			pte = swp_entry_to_pte(entry);
> +			if (pte_swp_soft_dirty(*src_pte))
> +				pte = pte_swp_mksoft_dirty(pte);
> +			if (pte_swp_uffd_wp(*src_pte))
> +				pte = pte_swp_mkuffd_wp(pte);
> +			set_pte_at(src_mm, addr, src_pte, pte);
> +		}

This needs to have the same logic as we now have in
copy_present_page(). The page *is* present and we can't copy the PTE
value hidden in a swap entry if we can't copy the PTE normally.

The code should be shared because nobody is going to remember about
this corner case.

Jason