linux-kernel - Re: [PATCH RFC 01/13] mm/hmm: Process pud swap entry without pud

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Message-ID: <20240307181233.GD9179@nvidia.com>
Date: Thu, 7 Mar 2024 14:12:33 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: peterx@...hat.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Matthew Wilcox <willy@...radead.org>,
	Mike Rapoport <rppt@...nel.org>,
	Christophe Leroy <christophe.leroy@...roup.eu>, x86@...nel.org,
	sparclinux@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Alistair Popple <apopple@...dia.com>
Subject: Re: [PATCH RFC 01/13] mm/hmm: Process pud swap entry without
 pud_huge()

On Wed, Mar 06, 2024 at 06:41:35PM +0800, peterx@...hat.com wrote:
> From: Peter Xu <peterx@...hat.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@...dia.com>
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/hmm.c | 7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.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() ?

Jason