linux-kernel - Re: [PATCH] x86/mm/ident_map: Use full gbpages in identity maps except on UV platform.

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Message-ID: <2f8d726a-9800-4068-9c0c-6c4a79d69a85@intel.com>
Date: Fri, 22 Mar 2024 10:40:37 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Steve Wahl <steve.wahl@....com>, Dave Hansen
 <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 linux-kernel@...r.kernel.org,
 Linux regressions mailing list <regressions@...ts.linux.dev>,
 Pavin Joseph <me@...injoseph.com>, stable@...r.kernel.org,
 Eric Hagberg <ehagberg@...il.com>, Simon Horman <horms@...ge.net.au>,
 Dave Young <dyoung@...hat.com>, Sarah Brofeldt <srhb@....dk>,
 Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>
Subject: Re: [PATCH] x86/mm/ident_map: Use full gbpages in identity maps
 except on UV platform.

On 3/22/24 10:31, Eric W. Biederman wrote:
>> I'd much rather add synthetic entries to the memory maps that have this
>> information than hack around it by assuming that things are within a
>> gigabyte.
> So this change is a partial revert of a change that broke kexec in
> existing configurations.  To fix a regression that breaks kexec.

Let's back up for a second:

 * Mapping extra memory on UV systems causes halts[1]
 * Mapping extra memory on UV systems breaks kexec (this thread)

So we're in a pickle.  I understand your concern for kexec.  But I'm
concerned that fixing the kexec problem will re-expose us to the [1]
problem.

Steve, can you explain a bit why this patch doesn't re-expose the kernel
to the [1] bug?

1. https://lore.kernel.org/all/20240126164841.170866-1-steve.wahl@hpe.com/