Re: [PATCH v1 4/7] arm64/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

[David Hildenbrand <david@redhat.com>]

On 17.03.22 18:58, Catalin Marinas wrote:
> On Thu, Mar 17, 2022 at 11:04:18AM +0100, David Hildenbrand wrote:
>> On 16.03.22 19:27, Catalin Marinas wrote:
>>> On Tue, Mar 15, 2022 at 03:18:34PM +0100, David Hildenbrand wrote:
>>>> @@ -909,12 +925,13 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
>>>>  /*
>>>>   * Encode and decode a swap entry:
>>>>   *	bits 0-1:	present (must be zero)
>>>> - *	bits 2-7:	swap type
>>>> + *	bits 2:		remember PG_anon_exclusive
>>>> + *	bits 3-7:	swap type
>>>>   *	bits 8-57:	swap offset
>>>>   *	bit  58:	PTE_PROT_NONE (must be zero)
>>>
>>> I don't remember exactly why we reserved bits 0 and 1 when, from the
>>> hardware perspective, it's sufficient for bit 0 to be 0 and the whole
>>> pte becomes invalid. We use bit 1 as the 'table' bit (when 0 at pmd
>>> level, it's a huge page) but we shouldn't check for this on a swap
>>> entry.
>>
>> You mean
>>
>> arch/arm64/include/asm/pgtable-hwdef.h:#define PTE_TABLE_BIT            (_AT(pteval_t, 1) << 1)
>>
>> right?
> 
> Yes.
> 
>> I wonder why it even exists, for arm64 I only spot:
>>
>> arch/arm64/include/asm/pgtable.h:#define pte_mkhuge(pte)                (__pte(pte_val(pte) & ~PTE_TABLE_BIT))
>>
>> I don't really see code that sets PTE_TABLE_BIT.
>>
>> Similarly, I don't see code that sets PMD_TABLE_BIT/PUD_TABLE_BIT/P4D_TABLE_BIT.
>> Most probably setting code is not using the defines,  that's why I'm not finding it.
> 
> It gets set as part of P*D_TYPE_TABLE via p*d_populate(). We use the
> P*D_TABLE_BIT mostly for checking whether it's a huge page or not (the
> arm64 hugetlbpage.c code).
> 

Makes sense, after digging into the arm arm, I agree that it should
be safe to reuse bit 1. I'll use this (yet untested) patch in v2:


From a48d08339574b7c42e0b032f0fc334872591744c Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@redhat.com>
Date: Thu, 17 Mar 2022 11:46:26 +0100
Subject: [PATCH] arm64/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE

Let's use bit 1, which should be irrelevant if the PTE is marked invalid
eiher way --  we really only care about bit 0.

Note that one alternative would be using one of the type bits: core-mm only
supports 5 bits, so there is no need to reserve space for 6.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 arch/arm64/include/asm/pgtable-prot.h |  1 +
 arch/arm64/include/asm/pgtable.h      | 19 ++++++++++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index b1e1b74d993c..fd6ddf14c190 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -14,6 +14,7 @@
  * Software defined PTE bits definition.
  */
 #define PTE_WRITE		(PTE_DBM)		 /* same as DBM (51) */
+#define PTE_SWP_EXCLUSIVE	(PTE_TABLE_BIT)		 /* only for swp ptes */
 #define PTE_DIRTY		(_AT(pteval_t, 1) << 55)
 #define PTE_SPECIAL		(_AT(pteval_t, 1) << 56)
 #define PTE_DEVMAP		(_AT(pteval_t, 1) << 57)
diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 94e147e5456c..c78994073cd0 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -402,6 +402,22 @@ static inline pgprot_t mk_pmd_sect_prot(pgprot_t prot)
 	return __pgprot((pgprot_val(prot) & ~PMD_TABLE_BIT) | PMD_TYPE_SECT);
 }
 
+#define __HAVE_ARCH_PTE_SWP_EXCLUSIVE
+static inline pte_t pte_swp_mkexclusive(pte_t pte)
+{
+	return set_pte_bit(pte, __pgprot(PTE_SWP_EXCLUSIVE));
+}
+
+static inline int pte_swp_exclusive(pte_t pte)
+{
+	return pte_val(pte) & PTE_SWP_EXCLUSIVE;
+}
+
+static inline pte_t pte_swp_clear_exclusive(pte_t pte)
+{
+	return clear_pte_bit(pte, __pgprot(PTE_SWP_EXCLUSIVE));
+}
+
 #ifdef CONFIG_NUMA_BALANCING
 /*
  * See the comment in include/linux/pgtable.h
@@ -908,7 +924,8 @@ static inline pmd_t pmdp_establish(struct vm_area_struct *vma,
 
 /*
  * Encode and decode a swap entry:
- *	bits 0-1:	present (must be zero)
+ *	bits 0:		present (must be zero)
+ *	bits 1:		remember PG_anon_exclusive
  *	bits 2-7:	swap type
  *	bits 8-57:	swap offset
  *	bit  58:	PTE_PROT_NONE (must be zero)
-- 
2.35.1


-- 
Thanks,

David / dhildenb