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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date: Sun, 24 Mar 2024 11:31:39 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Steve Wahl <steve.wahl@hpe.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org,
	Linux regressions mailing list <regressions@lists.linux.dev>,
	Pavin Joseph <me@pavinjoseph.com>, stable@vger.kernel.org,
	Eric Hagberg <ehagberg@gmail.com>,
	Simon Horman <horms@verge.net.au>,
	Eric Biederman <ebiederm@xmission.com>,
	Dave Young <dyoung@redhat.com>, Sarah Brofeldt <srhb@dbc.dk>,
	Russ Anderson <rja@hpe.com>, Dimitri Sivanich <sivanich@hpe.com>
Subject: Re: [PATCH] x86/mm/ident_map: Use full gbpages in identity maps
 except on UV platform.
Message-ID: <ZgABC1oQ9YJW6Bw3@gmail.com>
References: <20240322162135.3984233-1-steve.wahl@hpe.com>
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* Steve Wahl <steve.wahl@hpe.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@hpe.com>
> Fixes: d794734c9bbf ("x86/mm/ident_map: Use gbpages only where full GB page should be mapped.")
> Reported-by: Pavin Joseph <me@pavinjoseph.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