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Subject: Re: [PATCH v2 1/2] secretmem/gup: don't check if page is secretmem
 without reference
To: Mike Rapoport <rppt@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
References: <20210420131611.8259-1-rppt@kernel.org>
 <20210420131611.8259-2-rppt@kernel.org>
From: David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <95b7fa81-f72e-c63f-0456-4c25dee8a5eb@redhat.com>
Date: Tue, 20 Apr 2021 15:19:56 +0200
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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>, 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>, Michael Kerrisk <mtk.manpages@gmail.com>, Palmer Dabbelt <palmer@dabbelt.com>, Paul Walmsley <paul.walmsley@sifive.com>, Peter Zijlstra <peterz@infradead.org>, "Rafael J. Wysocki" <rjw@rjwysocki.net>, Rick Edgecombe <rick.p.edgecombe@intel.com>, Roman Gushchin <guro@fb.com>, Shakeel Butt <shakeelb@google.com>, Shuah Khan <shuah@kernel.org>, Thomas Gleixn
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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
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