Re: [PATCH mm-unstable v1 04/26] arm/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

David Hildenbrand
2023-02-08T14:12:06Z

On 07.02.23 01:32, Mark Brown wrote:
> On Fri, Jan 13, 2023 at 06:10:04PM +0100, David Hildenbrand wrote:
>> Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE by stealing one bit from the
>> offset. This reduces the maximum swap space per file to 64 GiB (was 128
>> GiB).
>>
>> While at it drop the PTE_TYPE_FAULT from __swp_entry_to_pte() which is
>> defined to be 0 and is rather confusing because we should be dealing
>> with "Linux PTEs" not "hardware PTEs". Also, properly mask the type in
>> __swp_entry().
>
> Today's -next (and at least back to Friday, older logs are unclear - I
> only noticed -next issues today) fails to NFS boot on an AT91SAM9G20-EK
> (an old ARMv5 platform) with multi_v5_defconfig, a bisect appears to
> point to this patch (20aae9eff5acd8f5 in today's -next) as the culprit.

It's been in -next for quite a while, thanks for the report!

>
> The failure happens at some point after starting userspace, the kernel
> starts spamming the console with messages in the form:
>
> get_swap_device: Bad swap file entry 10120d20
>

_swap_info_get() tells us that the swp type seems to be bad.
I assume we're dealing with a migration entry, if swap is disabled, and fail to
detect is_migration_entry() correctly because the type is messed up.

Could you give the following a test?

From 8c4bdbd9862f85782d5919d044c172b584063e83 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Wed, 8 Feb 2023 15:08:01 +0100
Subject: [PATCH] arm/mm: Fix swp type masking in __swp_entry()

We're masking with the number of type bits instead of the type mask, which
is obviously wrong.

Fixes: 20aae9eff5ac ("arm/mm: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE")
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
arch/arm/include/asm/pgtable.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
index 2e626e6da9a3..a58ccbb406ad 100644
--- a/arch/arm/include/asm/pgtable.h
+++ b/arch/arm/include/asm/pgtable.h
@@ -292,7 +292,7 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)

#define __swp_type(x) (((x).val >> __SWP_TYPE_SHIFT) & __SWP_TYPE_MASK)
#define __swp_offset(x) ((x).val >> __SWP_OFFSET_SHIFT)
-#define __swp_entry(type, offset) ((swp_entry_t) { (((type) & __SWP_TYPE_BITS) << __SWP_TYPE_SHIFT) | \
+#define __swp_entry(type, offset) ((swp_entry_t) { (((type) & __SWP_TYPE_MASK) << __SWP_TYPE_SHIFT) | \
((offset) << __SWP_OFFSET_SHIFT) })

#define __pte_to_swp_entry(pte) ((swp_entry_t) { pte_val(pte) })
--
2.39.1

--
Thanks,

David / dhildenb