linux-kernel - Re: [PATCH v1 05/12] mm/memory: detect writability in restore_exclusive_pte() through can_change_pte

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Message-ID: <Z5tLmYOQaZrdWQHN@phenom.ffwll.local>
Date: Thu, 30 Jan 2025 10:51:21 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
	nouveau@...ts.freedesktop.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jérôme Glisse <jglisse@...hat.com>,
	Jonathan Corbet <corbet@....net>, Alex Shi <alexs@...nel.org>,
	Yanteng Si <si.yanteng@...ux.dev>,
	Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...nel.org>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Peter Xu <peterx@...hat.com>, Alistair Popple <apopple@...dia.com>,
	Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH v1 05/12] mm/memory: detect writability in
 restore_exclusive_pte() through can_change_pte_writable()

On Wed, Jan 29, 2025 at 12:54:03PM +0100, David Hildenbrand wrote:
> Let's do it just like mprotect write-upgrade or during NUMA-hinting
> faults on PROT_NONE PTEs: detect if the PTE can be writable by using
> can_change_pte_writable().
> 
> Set the PTE only dirty if the folio is dirty: we might not
> necessarily have a write access, and setting the PTE writable doesn't
> require setting the PTE dirty.

Not sure whether there's much difference in practice, since a device
exclusive access means a write, so the folio better be dirty (unless we
aborted halfway through). But then I couldn't find the code in nouveau to
do that, so now I'm confused.
-Sima

> With this change in place, there is no need to have separate
> readable and writable device-exclusive entry types, and we'll merge
> them next separately.
> 
> Note that, during fork(), we first convert the device-exclusive entries
> back to ordinary PTEs, and we only ever allow conversion of writable
> PTEs to device-exclusive -- only mprotect can currently change them to
> readable-device-exclusive. Consequently, we always expect
> PageAnonExclusive(page)==true and can_change_pte_writable()==true,
> unless we are dealing with soft-dirty tracking or uffd-wp. But reusing
> can_change_pte_writable() for now is cleaner.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  mm/memory.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 03efeeef895a..db38d6ae4e74 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -725,18 +725,21 @@ static void restore_exclusive_pte(struct vm_area_struct *vma,
>  	struct folio *folio = page_folio(page);
>  	pte_t orig_pte;
>  	pte_t pte;
> -	swp_entry_t entry;
>  
>  	orig_pte = ptep_get(ptep);
>  	pte = pte_mkold(mk_pte(page, READ_ONCE(vma->vm_page_prot)));
>  	if (pte_swp_soft_dirty(orig_pte))
>  		pte = pte_mksoft_dirty(pte);
>  
> -	entry = pte_to_swp_entry(orig_pte);
>  	if (pte_swp_uffd_wp(orig_pte))
>  		pte = pte_mkuffd_wp(pte);
> -	else if (is_writable_device_exclusive_entry(entry))
> -		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
> +
> +	if ((vma->vm_flags & VM_WRITE) &&
> +	    can_change_pte_writable(vma, address, pte)) {
> +		if (folio_test_dirty(folio))
> +			pte = pte_mkdirty(pte);
> +		pte = pte_mkwrite(pte, vma);
> +	}
>  
>  	VM_BUG_ON_FOLIO(pte_write(pte) && (!folio_test_anon(folio) &&
>  					   PageAnonExclusive(page)), folio);
> -- 
> 2.48.1
> 

-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch