linux-kernel - Re: [PATCH 2/9] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

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Date:   Thu, 21 Jan 2021 13:52:30 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Chinwen Chang <chinwen.chang@...iatek.com>,
        Huang Ying <ying.huang@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Jann Horn <jannh@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michal Koutný <mkoutny@...e.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>, Shaohua Li <shli@...com>,
        Shawn Anastasio <shawn@...stas.io>,
        Steven Rostedt <rostedt@...dmis.org>,
        Steven Price <steven.price@....com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        Adam Ruprecht <ruprecht@...gle.com>,
        Cannon Matthews <cannonmatthews@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Oliver Upton <oupton@...gle.com>
Subject: Re: [PATCH 2/9] hugetlb/userfaultfd: Forbid huge pmd sharing when
 uffd enabled

On Fri, Jan 15, 2021 at 11:04:44AM -0800, Axel Rasmussen wrote:

[...]

> @@ -947,4 +948,15 @@ static inline __init void hugetlb_cma_check(void)
>  }
>  #endif
>  
> +static inline bool want_pmd_share(struct vm_area_struct *vma)
> +{
> +#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
> +	if (uffd_disable_huge_pmd_share(vma))
> +		return false;
> +	return true;
> +#else
> +	return false;
> +#endif
> +}

I got syzbot complains about this chunk when compile some other archs outside
x86, probably because CONFIG_ARCH_WANT_HUGE_PMD_SHARE can be defined without
CONFIG_USERFAULTFD.  I don't know why it didn't complain here, so just a fyi
that I've pushed some new version to the online repo so that they should have
been fixed.  For example, for this patch:

https://github.com/xzpeter/linux/commit/0fd01db2c9ac3ab8600f3f7df23cf28fddcfee1b

static inline bool want_pmd_share(struct vm_area_struct *vma)
{
#ifdef CONFIG_USERFAULTFD
	if (uffd_disable_huge_pmd_share(vma))
		return false;
#endif

#ifdef CONFIG_ARCH_WANT_HUGE_PMD_SHARE
	return true;
#else
	return false;
#endif
}

Thanks,

-- 
Peter Xu