linux-kernel - Re: [PATCH] x86/mm: lower MAP_32BIT begin to reduce heap collisions

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Message-ID: <c69341d2-bf01-48df-9eeb-e346f72c9409@intel.com>
Date: Mon, 25 Aug 2025 07:02:37 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Simon Liebold <simonlie@...zon.de>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>
Cc: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Mark Brown <broonie@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Helge Deller <deller@....de>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Simon Liebold <lieboldsimonpaul@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: lower MAP_32BIT begin to reduce heap collisions

On 8/25/25 03:48, Simon Liebold wrote:
> Commit 03475167fda5 ("x86: Increase brk randomness entropy for 64-bit
> systems") increased the brk randomness from 32 MiB to 1 GiB. MAP_32BIT
> looks between 1 GiB and 2 GiB for an unmapped area. Depending on the
> randomization, a heap starting high enough and being big enough can use
> up all the area that MAP_32BIT looks at, leading to allocation failures.

Isn't that still a really unreasonably gigantic heap?

Would you mind posting some actual /proc/$pid/maps output from one of
the failure cases?