[PATCH] x86/mm: Fix boot with some memory above MAXMEM

[PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Kirill A. Shutemov]

A 5-level paging capable machine can have memory above 46-bit in the
physical address space. This memory is only addressable in the 5-level
paging mode: we don't have enough virtual address space to create direct
mapping for such memory in the 4-level paging mode.

Currently, we fail boot completely: NULL pointer dereference in
subsection_map_init().

Skip creating a memblock for such memory instead and notify user that
some memory is not addressable.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: stable@vger.kernel.org # v4.14
---

Tested with a hacked QEMU: https://gist.github.com/kiryl/d45eb54110944ff95e544972d8bdac1d

---
 arch/x86/kernel/e820.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index c5399e80c59c..022fe1de8940 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1307,7 +1307,14 @@ void __init e820__memblock_setup(void)
 		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
 			continue;
 
-		memblock_add(entry->addr, entry->size);
+		if (entry->addr >= MAXMEM || end >= MAXMEM)
+			pr_err_once("Some physical memory is not addressable in the paging mode.\n");
+
+		if (entry->addr >= MAXMEM)
+			continue;
+
+		end = min_t(u64, end, MAXMEM - 1);
+		memblock_add(entry->addr, end - entry->addr);
 	}
 
 	/* Throw away partial pages: */
-- 
2.26.2

Re: [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Dave Hansen]

On 5/11/20 9:37 AM, Kirill A. Shutemov wrote:
> -		memblock_add(entry->addr, entry->size);
> +		if (entry->addr >= MAXMEM || end >= MAXMEM)
> +			pr_err_once("Some physical memory is not addressable in the paging mode.\n");

Hi Kirill,

Thanks for fixing this!

Could we make the pr_err() a bit more informative, though?  It would be
nice to print out how much memory (or which addresses at least) are
being thrown away.

I was also thinking that it would be handy to tell folks how to rectify
the situation.  Should we perhaps dump out the runtime status of
X86_FEATURE_LA57?

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Kirill A. Shutemov]

On Mon, May 11, 2020 at 09:43:30AM -0700, Dave Hansen wrote:
> On 5/11/20 9:37 AM, Kirill A. Shutemov wrote:
> > -		memblock_add(entry->addr, entry->size);
> > +		if (entry->addr >= MAXMEM || end >= MAXMEM)
> > +			pr_err_once("Some physical memory is not addressable in the paging mode.\n");
> 
> Hi Kirill,
> 
> Thanks for fixing this!
> 
> Could we make the pr_err() a bit more informative, though?  It would be
> nice to print out how much memory (or which addresses at least) are
> being thrown away.
> 
> I was also thinking that it would be handy to tell folks how to rectify
> the situation.  Should we perhaps dump out the runtime status of
> X86_FEATURE_LA57?

Something like this (incremental patch)?

diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
index 022fe1de8940..172b4244069f 100644
--- a/arch/x86/kernel/e820.c
+++ b/arch/x86/kernel/e820.c
@@ -1280,8 +1280,8 @@ void __init e820__memory_setup(void)
 
 void __init e820__memblock_setup(void)
 {
+	u64 size, end, not_addressable = 0;
 	int i;
-	u64 end;
 
 	/*
 	 * The bootstrap memblock region count maximum is 128 entries
@@ -1307,16 +1307,24 @@ void __init e820__memblock_setup(void)
 		if (entry->type != E820_TYPE_RAM && entry->type != E820_TYPE_RESERVED_KERN)
 			continue;
 
-		if (entry->addr >= MAXMEM || end >= MAXMEM)
-			pr_err_once("Some physical memory is not addressable in the paging mode.\n");
-
-		if (entry->addr >= MAXMEM)
+		if (entry->addr >= MAXMEM) {
+			not_addressable += entry->size;
 			continue;
+		}
 
 		end = min_t(u64, end, MAXMEM - 1);
+		size = end - entry->addr;
+		not_addressable += entry->size - size;
 		memblock_add(entry->addr, end - entry->addr);
 	}
 
+	if (not_addressable) {
+		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
+		       not_addressable >> 20);
+		if (!pgtable_l5_enabled())
+			pr_err("Consider enabling 5-level paging\n");
+	}
+
 	/* Throw away partial pages: */
 	memblock_trim_memory(PAGE_SIZE);
 
-- 
 Kirill A. Shutemov

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Kirill A. Shutemov]

On Mon, May 11, 2020 at 07:37:06PM +0300, Kirill A. Shutemov wrote:
> A 5-level paging capable machine can have memory above 46-bit in the
> physical address space. This memory is only addressable in the 5-level
> paging mode: we don't have enough virtual address space to create direct
> mapping for such memory in the 4-level paging mode.
> 
> Currently, we fail boot completely: NULL pointer dereference in
> subsection_map_init().
> 
> Skip creating a memblock for such memory instead and notify user that
> some memory is not addressable.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Cc: stable@vger.kernel.org # v4.14
> ---
> 
> Tested with a hacked QEMU: https://gist.github.com/kiryl/d45eb54110944ff95e544972d8bdac1d

BTW, I was only able to boot with legacy SeaBIOS, not with OVMF. No idea
why.

-- 
 Kirill A. Shutemov

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Dave Hansen]

On 5/11/20 10:04 AM, Kirill A. Shutemov wrote:
> +	if (not_addressable) {
> +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
> +		       not_addressable >> 20);
> +		if (!pgtable_l5_enabled())
> +			pr_err("Consider enabling 5-level paging\n");
> +	}

Looks sane to me.  Definitely good enough until we get the first bug
reports from an end user about how they screwed this up in practice.

For the aggregate patch:

Reviewed-by: Dave Hansen <dave.hansen@intel.com>

BTW, it's a shame that 0day and friends can't find stuff like this.  I
have the feeling we have more bugs like this coming.

RE: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Luck, Tony]

> +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
> +		       not_addressable >> 20);

Is "MB" the right unit for this. The problem seems to happen for systems with >64TB ... I doubt
the unaddressable memory is just a couple of MBbytes

-Tony

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Kirill A. Shutemov]

On Mon, May 11, 2020 at 06:10:12PM +0000, Luck, Tony wrote:
> > +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
> > +		       not_addressable >> 20);
> 
> Is "MB" the right unit for this. The problem seems to happen for systems with >64TB ... I doubt
> the unaddressable memory is just a couple of MBbytes

Change it to GB?

-- 
 Kirill A. Shutemov

RE: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Luck, Tony]

>> > +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
>> > +		       not_addressable >> 20);
>> 
>> Is "MB" the right unit for this. The problem seems to happen for systems with >64TB ... I doubt
>> the unaddressable memory is just a couple of MBbytes
>
> Change it to GB?

I think it would be more readable.

[Maybe Linux needs a magic %p{something} that does auto-sizing to print in the most appropriate out of KB, MB, GB, TB, PB?]

-Tony

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Andy Shevchenko]

On Mon, May 11, 2020 at 06:58:21PM +0000, Luck, Tony wrote:
> >> > +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
> >> > +		       not_addressable >> 20);
> >> 
> >> Is "MB" the right unit for this. The problem seems to happen for systems with >64TB ... I doubt
> >> the unaddressable memory is just a couple of MBbytes
> >
> > Change it to GB?
> 
> I think it would be more readable.
> 
> [Maybe Linux needs a magic %p{something} that does auto-sizing to print in the most appropriate out of KB, MB, GB, TB, PB?]

We have one in string_helpers.c.

-- 
With Best Regards,
Andy Shevchenko

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Luck, Tony]

On Tue, May 12, 2020 at 12:19:25AM +0300, Andy Shevchenko wrote:
> On Mon, May 11, 2020 at 06:58:21PM +0000, Luck, Tony wrote:
> > >> > +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
> > >> > +		       not_addressable >> 20);
> > >> 
> > >> Is "MB" the right unit for this. The problem seems to happen for systems with >64TB ... I doubt
> > >> the unaddressable memory is just a couple of MBbytes
> > >
> > > Change it to GB?
> > 
> > I think it would be more readable.
> > 
> > [Maybe Linux needs a magic %p{something} that does auto-sizing to print in the most appropriate out of KB, MB, GB, TB, PB?]
> 
> We have one in string_helpers.c.

Ah, nice.  So:

#include <linux/string_helpers.h>


	char	tmp[10]; /* Bother, no #define for this, just a comment in string_helpers.c */


	string_get_size(not_addressable, 1, STRING_UNITS_2, tmp, sizeof(tmp);

	pr_err("%s of physical memory is not addressable in the paging mode\n", tmp);

-Tony

Re: [linux-drivers-review] [PATCH] x86/mm: Fix boot with some memory above MAXMEM

[Kirill A. Shutemov]

On Mon, May 11, 2020 at 05:50:01PM -0700, Luck, Tony wrote:
> On Tue, May 12, 2020 at 12:19:25AM +0300, Andy Shevchenko wrote:
> > On Mon, May 11, 2020 at 06:58:21PM +0000, Luck, Tony wrote:
> > > >> > +		pr_err("%lldMB of physical memory is not addressable in the paging mode\n",
> > > >> > +		       not_addressable >> 20);
> > > >> 
> > > >> Is "MB" the right unit for this. The problem seems to happen for systems with >64TB ... I doubt
> > > >> the unaddressable memory is just a couple of MBbytes
> > > >
> > > > Change it to GB?
> > > 
> > > I think it would be more readable.
> > > 
> > > [Maybe Linux needs a magic %p{something} that does auto-sizing to print in the most appropriate out of KB, MB, GB, TB, PB?]
> > 
> > We have one in string_helpers.c.
> 
> Ah, nice.  So:
> 
> #include <linux/string_helpers.h>
> 
> 
> 	char	tmp[10]; /* Bother, no #define for this, just a comment in string_helpers.c */
> 
> 
> 	string_get_size(not_addressable, 1, STRING_UNITS_2, tmp, sizeof(tmp);
> 
> 	pr_err("%s of physical memory is not addressable in the paging mode\n", tmp);

I've already submitted the patch upstream. I'll add it if a new revision
is needed.

-- 
 Kirill A. Shutemov