linux-kernel - Re: v4.4-rc8 based tip tree kernel hangs on boot.

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Date:	Tue, 19 Jan 2016 15:42:41 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, linux-efi@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: v4.4-rc8 based tip tree kernel hangs on boot.

On Tue, 19 Jan, at 04:17:15PM, Srikar Dronamraju wrote:
> * Matt Fleming <matt@...eblueprint.co.uk> [2016-01-18 15:40:01]:
> 
> > On Sun, 17 Jan, at 11:02:48PM, Srikar Dronamraju wrote:
> > > 
> > > Unfortunately even with your below patch on top of c9f2a9a ("x86/efi:
> > > Hoist page table switching code into efi_call_virt()" also fails to boot
> > > with and without CONFIG_DEBUG_WX=y. I have added CONFIG_EFI_PGT_DUMP.
> > 
> > What about if you comment out the call to efi_dump_pagetable()? What
> > if you return early from efi_sync_low_kernel_mappings(), e.g.
> 
> This doesnt help but ....
> 
> > 
> > diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
> > index 71121a9d06cf..e1f8ebb88f84 100644
> > --- a/arch/x86/platform/efi/efi_64.c
> > +++ b/arch/x86/platform/efi/efi_64.c
> > @@ -177,6 +177,8 @@ void efi_sync_low_kernel_mappings(void)
> >  		init_mm.pgd + pgd_index(PAGE_OFFSET),
> >  		sizeof(pgd_t) * num_pgds);
> > 
> > +	return;
> > +
> 
> This alone makes it boot. dmesg below
> 
> i.e git checkout tip/x86/efi
> +
> git am <your patch> + <the above fixlet>
> 

OK could you add efi=debug to your kernel cmdline and try this patch
instead of the one that returns early from
efi_sync_low_kernel_mappings()?

I also pushed out all the changes to the following branch if that's
easier to pull and build,

  https://git.kernel.org/cgit/linux/kernel/git/mfleming/efi.git/log/?h=bug/efi/srikar

I have no idea why any of these entries would be empty, but this is
the only thing I can imagine could go wrong with this code,

diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index 71121a9d06cf..9b610aa17683 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -192,6 +192,9 @@ void efi_sync_low_kernel_mappings(void)
 	pgd_efi = efi_pgd + pgd_index(PAGE_OFFSET);
 	pgd_k = pgd_offset_k(PAGE_OFFSET);
 
+	if (WARN_ON_ONCE(pgd_none(*pgd_efi) || pgd_none(*pgd_k)))
+		return;
+
 	num_entries = pgd_index(EFI_VA_END) - pgd_index(PAGE_OFFSET);
 	memcpy(pgd_efi, pgd_k, sizeof(pgd_t) * num_entries);
 
@@ -203,17 +206,26 @@ void efi_sync_low_kernel_mappings(void)
 	BUILD_BUG_ON((EFI_VA_END & ~PUD_MASK) != 0);
 
 	pgd_efi = efi_pgd + pgd_index(EFI_VA_END);
-	pud_efi = pud_offset(pgd_efi, 0);
-
 	pgd_k = pgd_offset_k(EFI_VA_END);
+
+	if (WARN_ON_ONCE(pgd_none(*pgd_efi) || pgd_none(*pgd_k)))
+		return;
+
+	pud_efi = pud_offset(pgd_efi, 0);
 	pud_k = pud_offset(pgd_k, 0);
 
+	if (WARN_ON_ONCE(pud_none(*pud_efi) || pud_none(*pud_k)))
+		return;
+
 	num_entries = pud_index(EFI_VA_END);
 	memcpy(pud_efi, pud_k, sizeof(pud_t) * num_entries);
 
 	pud_efi = pud_offset(pgd_efi, EFI_VA_START);
 	pud_k = pud_offset(pgd_k, EFI_VA_START);
 
+	if (WARN_ON_ONCE(pud_none(*pud_efi) || pud_none(*pud_k)))
+		return;
+
 	num_entries = PTRS_PER_PUD - pud_index(EFI_VA_START);
 	memcpy(pud_efi, pud_k, sizeof(pud_t) * num_entries);
 }