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Date: Fri, 8 Mar 2024 14:50:20 +0800
From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@nvidia.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,
	Alistair Popple <apopple@nvidia.com>
Subject: Re: [PATCH RFC 01/13] mm/hmm: Process pud swap entry without
 pud_huge()
Message-ID: <Zeq1LNValPosuWgw@x1n>
References: <20240306104147.193052-1-peterx@redhat.com>
 <20240306104147.193052-2-peterx@redhat.com>
 <20240307181233.GD9179@nvidia.com>
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In-Reply-To: <20240307181233.GD9179@nvidia.com>

On Thu, Mar 07, 2024 at 02:12:33PM -0400, Jason Gunthorpe wrote:
> On Wed, Mar 06, 2024 at 06:41:35PM +0800, peterx@redhat.com wrote:
> > From: Peter Xu <peterx@redhat.com>
> > 
> > Swap pud entries do not always return true for pud_huge() for all archs.
> > x86 and sparc (so far) allow it, but all the rest do not accept a swap
> > entry to be reported as pud_huge().  So it's not safe to check swap entries
> > within pud_huge().  Check swap entries before pud_huge(), so it should be
> > always safe.
> > 
> > This is the only place in the kernel that (IMHO, wrongly) relies on
> > pud_huge() to return true on pud swap entries.  The plan is to cleanup
> > pXd_huge() to only report non-swap mappings for all archs.
> > 
> > Cc: Alistair Popple <apopple@nvidia.com>
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  mm/hmm.c | 7 +------
> >  1 file changed, 1 insertion(+), 6 deletions(-)
> 
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> 
> > @@ -424,7 +424,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
> >  	walk->action = ACTION_CONTINUE;
> >  
> >  	pud = READ_ONCE(*pudp);
> > -	if (pud_none(pud)) {
> > +	if (pud_none(pud) || !pud_present(pud)) {
> 
> Isn't this a tautology? pud_none always implies !present() ?

Hmm yes I think so, afact, it should be "all=none+swap+present". I still
remember I missed that once previously, it's not always obvious when
preparing such patches. :( I'll simplify this and also on patch 3.

Thanks,

-- 
Peter Xu