From: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, "H. Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>, Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@amacapital.net>, Michal Hocko <mhocko@suse.com>,
        linux-arch@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH 26/26] x86/mm: allow to have userspace mappings above 47-bits
In-Reply-To: <20170313055020.69655-27-kirill.shutemov@linux.intel.com>
References: <20170313055020.69655-1-kirill.shutemov@linux.intel.com> <20170313055020.69655-27-kirill.shutemov@linux.intel.com>
Date: Fri, 17 Mar 2017 23:23:54 +0530
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"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> writes:

> On x86, 5-level paging enables 56-bit userspace virtual address space.
> Not all user space is ready to handle wide addresses. It's known that
> at least some JIT compilers use higher bits in pointers to encode their
> information. It collides with valid pointers with 5-level paging and
> leads to crashes.
>
> To mitigate this, we are not going to allocate virtual address space
> above 47-bit by default.
>
> But userspace can ask for allocation from full address space by
> specifying hint address (with or without MAP_FIXED) above 47-bits.
>
> If hint address set above 47-bit, but MAP_FIXED is not specified, we try
> to look for unmapped area by specified address. If it's already
> occupied, we look for unmapped area in *full* address space, rather than
> from 47-bit window.
>
> This approach helps to easily make application's memory allocator aware
> about large address space without manually tracking allocated virtual
> address space.
>

So if I have done a successful mmap which returned > 128TB what should a
following mmap(0,...) return ? Should that now search the *full* address
space or below 128TB ?

-aneesh