Re: POWER: Unexpected fault when writing to brk-allocated memory

From: Florian Weimer <fweimer@redhat.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>,
	Nicholas Piggin <npiggin@gmail.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	linuxppc-dev@lists.ozlabs.org, linux-mm <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@amacapital.net>,
	Dave Hansen <dave.hansen@intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-arch@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: POWER: Unexpected fault when writing to brk-allocated memory
Date: Tue, 7 Nov 2017 12:26:12 +0100	[thread overview]
Message-ID: <c5586546-1e7e-0f0f-a8b3-680fadb38dcf@redhat.com> (raw)
In-Reply-To: <20171107111543.ep57evfxxbwwlhdh@node.shutemov.name>

On 11/07/2017 12:15 PM, Kirill A. Shutemov wrote:

>> First of all, using addr and MAP_FIXED to develop our heuristic can
>> never really give unchanged ABI. It's an in-band signal. brk() is a
>> good example that steadily keeps incrementing address, so depending
>> on malloc usage and address space randomization, you will get a brk()
>> that ends exactly at 128T, then the next one will be >
>> DEFAULT_MAP_WINDOW, and it will switch you to 56 bit address space.
> 
> No, it won't. You will hit stack first.

That's not actually true on POWER in some cases.  See the process maps I 
posted here:

   <https://marc.info/?l=linuxppc-embedded&m=150988538106263&w=2>

Thanks,
Florian