linux-kernel - Re: [PATCH] x86/mm: fix _pgd

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Message-ID: <2025418174638-aAKP_lnKYqenWy7u-arkamar@atlas.cz>
Date: Fri, 18 Apr 2025 19:46:38 +0200
From: Petr Vaněk <arkamar@...as.cz>
To: Juergen Gross <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH] x86/mm: fix _pgd_alloc() for Xen PV mode

On Thu, Apr 17, 2025 at 04:48:08PM +0200, Juergen Gross wrote:
> Recently _pgd_alloc() was switched from using __get_free_pages() to
> pagetable_alloc_noprof(), which might return a compound page in case
> the allocation order is larger than 0.
> 
> On x86 this will be the case if CONFIG_MITIGATION_PAGE_TABLE_ISOLATION
> is set, even if PTI has been disabled at runtime.
> 
> When running as a Xen PV guest (this will always disable PTI), using
> a compound page for a PGD will result in VM_BUG_ON_PGFLAGS being
> triggered when the Xen code tries to pin the PGD.

> Fix the Xen issue together with the not needed 8k allocation for a
> PGD with PTI disabled by using a variable holding the PGD allocation
> order in case CONFIG_MITIGATION_PAGE_TABLE_ISOLATION is set.
> 
> Reported-by: Petr Vaněk <arkamar@...as.cz>
> Fixes: a9b3c355c2e6 ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}")
> Cc: stable@...r.kernel.org
> Signed-off-by: Juergen Gross <jgross@...e.com>

I have runtime tested this patch, and it fixes the reported issue. The
following trailers can be appended to the commit message (as per [1]):

Closes: https://lore.kernel.org/lkml/202541612720-Z_-deOZTOztMXHBh-arkamar@atlas.cz/
Tested-by: Petr Vaněk <arkamar@...as.cz>

Cheers,
Petr

[1] https://docs.kernel.org/process/5.Posting.html#patch-formatting-and-changelogs