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Subject: Re: [PATCHv3 33/33] mm, x86: introduce PR_SET_MAX_VADDR and
 PR_GET_MAX_VADDR
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
        Linus Torvalds <torvalds@linux-foundation.org>
References: <20170217141328.164563-1-kirill.shutemov@linux.intel.com>
 <20170217141328.164563-34-kirill.shutemov@linux.intel.com>
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 <20170220131515.GA9502@node.shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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        Linux API <linux-api@vger.kernel.org>
From: Dave Hansen <dave.hansen@intel.com>
Message-ID: <0d05ac45-a139-6f8e-f98b-71876fbb509d@intel.com>
Date: Tue, 21 Feb 2017 12:46:55 -0800
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Let me make sure I'm grokking what you're trying to do here.

On 02/20/2017 05:15 AM, Kirill A. Shutemov wrote:
> +/* MPX cannot handle addresses above 47-bits yet. */
> +unsigned long mpx_unmapped_area_check(unsigned long addr, unsigned long len,
> +		unsigned long flags)
> +{
> +	if (!kernel_managing_mpx_tables(current->mm))
> +		return addr;
> +	if (addr + len <= DEFAULT_MAP_WINDOW)
> +		return addr;

At this point, we know MPX management is on and the hint is for memory
above DEFAULT_MAP_WINDOW?

> +	if (flags & MAP_FIXED)
> +		return -ENOMEM;

... and if it's a MAP_FIXED request, fail it.

> +	if (len > DEFAULT_MAP_WINDOW)
> +		return -ENOMEM;

What is this case for?  If addr+len wraps?

> +	/* Look for unmap area within DEFAULT_MAP_WINDOW */
> +	return 0;
> +}

Otherwise, blow away the hint, which we know is high and needs to
be discarded?