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

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Message-ID: <ZgABC1oQ9YJW6Bw3@gmail.com>
Date: Sun, 24 Mar 2024 11:31:39 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Steve Wahl <steve.wahl@....com>
Cc: 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>,
	Eric Biederman <ebiederm@...ssion.com>,
	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.


* Steve Wahl <steve.wahl@....com> wrote:

> Some systems have ACPI tables that don't include everything that needs
> to be mapped for a successful kexec.  These systems rely on identity
> maps that include the full gigabyte surrounding any smaller region
> requested for kexec success.  Without this, they fail to kexec and end
> up doing a full firmware reboot.
> 
> So, reduce the use of GB pages only on systems where this is known to
> be necessary (specifically, UV systems).
> 
> Signed-off-by: Steve Wahl <steve.wahl@....com>
> Fixes: d794734c9bbf ("x86/mm/ident_map: Use gbpages only where full GB page should be mapped.")
> Reported-by: Pavin Joseph <me@...injoseph.com>

Sigh, why was d794734c9bbf marked for a -stable backport? The commit 
never explains ...

If it's broken, it should be reverted - instead of trying to partially 
revert and then maybe break some other systems.

When there's boot breakage with new patches, we back out the bad patch 
and re-try in 99.9% of the cases.

Thanks,

	Ingo