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

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Message-ID: <ZgHO8T_dfrzPBTaZ@swahl-home.5wahls.com>
Date: Mon, 25 Mar 2024 14:22:25 -0500
From: Steve Wahl <steve.wahl@....com>
To: Ingo Molnar <mingo@...nel.org>
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>,
        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.

On Sun, Mar 24, 2024 at 11:31:39AM +0100, Ingo Molnar wrote:
> 
> * 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.

Fine with me, especially as you've already done the revert. :-)

I will create a new patch that combines the two.  If you have any
specific actions you'd like to be sure I do for this, let me know.

Thanks,

--> Steve Wahl
-- 
Steve Wahl, Hewlett Packard Enterprise