Re: [BUG] WARNING: CPU: 3 PID: 1 at mm/debug_vm_pgtable.c:493

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>,
	LKML <linux-kernel@vger.kernel.org>,
	Gavin Shan <gshan@redhat.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	"the arch/x86 maintainers" <x86@kernel.org>
Subject: Re: [BUG] WARNING: CPU: 3 PID: 1 at mm/debug_vm_pgtable.c:493
Date: Tue, 12 Oct 2021 12:15:40 -0700	[thread overview]
Message-ID: <CAHk-=wg6fw130AkO72GPFow9PHvP9odnC5LZ0UaY9bJQuF-C5A@mail.gmail.com> (raw)
In-Reply-To: <20211012145540.343541e9@gandalf.local.home>

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On Tue, Oct 12, 2021 at 11:55 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
>
> I saved off that warning from a different branch, and looking at the dmesg,
> it is:
>
>   Linux version 5.14.0-rc2-test+
>
> Which shows this happened on 5.14 as well (and explains why the line
> numbers are different).

Ok, yes, that's the exact same "pud_set_huge()" failure.

So it's the same issue, and has been going on for a while, and
presumably very few people end up enabling DEBUG_VM_PGTABLE (together
with it being hard to trigger).

Or it's something specific to your setup, but that sounds unlikely.

You might add some debugging to the x86 pud_set_huge() function,
something like the attached (obviously entirely untested) patch.

           Linus

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 arch/x86/mm/pgtable.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 3481b35cb4ec..0a3d67ec73d4 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -706,12 +706,16 @@ int pud_set_huge(pud_t *pud, phys_addr_t addr, pgprot_t prot)
 
 	mtrr = mtrr_type_lookup(addr, addr + PUD_SIZE, &uniform);
 	if ((mtrr != MTRR_TYPE_INVALID) && (!uniform) &&
-	    (mtrr != MTRR_TYPE_WRBACK))
+	    (mtrr != MTRR_TYPE_WRBACK)) {
+		pr_debug("mtrr_type_lookup() returned %d (%d)\n", mtrr, uniform);
 		return 0;
+	}
 
 	/* Bail out if we are we on a populated non-leaf entry: */
-	if (pud_present(*pud) && !pud_huge(*pud))
+	if (pud_present(*pud) && !pud_huge(*pud)) {
+		pr_debug("pud is already present (%lx)\n", (unsigned long)pud_val(*pud));
 		return 0;
+	}
 
 	set_pte((pte_t *)pud, pfn_pte(
 		(u64)addr >> PAGE_SHIFT,
