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Date: Thu, 7 Mar 2024 16:08:26 -0400
From: Jason Gunthorpe <jgg@nvidia.com>
To: peterx@redhat.com
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Muchun Song <muchun.song@linux.dev>,
	Matthew Wilcox <willy@infradead.org>,
	Mike Rapoport <rppt@kernel.org>,
	Christophe Leroy <christophe.leroy@csgroup.eu>, x86@kernel.org,
	sparclinux@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH RFC 03/13] mm/gup: Check p4d presence before going on
Message-ID: <20240307200826.GE9179@nvidia.com>
References: <20240306104147.193052-1-peterx@redhat.com>
 <20240306104147.193052-4-peterx@redhat.com>
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On Wed, Mar 06, 2024 at 06:41:37PM +0800, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
> 
> Currently there should have no p4d swap entries so it may not matter much,
> however this may help us to rule out swap entries in pXd_huge() API, which
> will include p4d_huge().  The p4d_present() checks make it 100% clear that
> we won't rely on p4d_huge() for swap entries.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  mm/gup.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

>  	p4dp = p4d_offset(pgdp, address);
>  	p4d = READ_ONCE(*p4dp);
> -	if (p4d_none(p4d))
> +	if (p4d_none(p4d) || !p4d_present(p4d))
>  		return no_page_table(vma, flags);

Like the other place I think this makes more sense as

    if (!p4_present(p4d))
  		return no_page_table(vma, flags);

Since none can never be present.

IOW if the following code doesn't decode swap entries then it should
be guarded with a chceck on present not none..

Jason