Re: [PATCH v2 1/2] secretmem/gup: don't check if page is secretmem without reference

From: David Hildenbrand <david@redhat.com>
To: Mike Rapoport <rppt@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Christopher Lameter <cl@linux.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	James Bottomley <jejb@linux.ibm.com>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Matthew Wilcox <willy@infradead.org>,
	Matthew Garrett <mjg59@srcf.ucam.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Michal Hocko <mhocko@suse.com>,
	Mike Rapoport <rppt@linux.ibm.com>,
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	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Peter Zijlstra <peterz@infradead.org>,
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	Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
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	linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
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Subject: Re: [PATCH v2 1/2] secretmem/gup: don't check if page is secretmem without reference
Date: Tue, 20 Apr 2021 15:19:56 +0200	[thread overview]
Message-ID: <95b7fa81-f72e-c63f-0456-4c25dee8a5eb@redhat.com> (raw)
In-Reply-To: <20210420131611.8259-2-rppt@kernel.org>

On 20.04.21 15:16, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> The check in gup_pte_range() whether a page belongs to a secretmem mapping
> is performed before grabbing the page reference.
> 
> To avoid potential race move the check after try_grab_compound_head().
> 
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>   mm/gup.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index c3a17b189064..4b58c016e949 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2080,13 +2080,13 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>   		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
>   		page = pte_page(pte);
>   
> -		if (page_is_secretmem(page))
> -			goto pte_unmap;
> -
>   		head = try_grab_compound_head(page, 1, flags);
>   		if (!head)
>   			goto pte_unmap;
>   
> +		if (page_is_secretmem(page))
> +			goto pte_unmap;
> +

Looking at the hunk below, I wonder if you're missing a put_compound_head().

(also, I'd do if unlikely(page_is_secretmem()) but that's a different 
discussion)

>   		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
>   			put_compound_head(head, 1, flags);
>   			goto pte_unmap;
> 

-- 
Thanks,

David / dhildenb