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

[David Hildenbrand]

On 21.03.22 15:38, Will Deacon wrote:
> On Wed, Mar 16, 2022 at 06:27:01PM +0000, Catalin Marinas wrote:
>> On Tue, Mar 15, 2022 at 03:18:34PM +0100, David Hildenbrand wrote:
>>> diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
>>> index b1e1b74d993c..62e0ebeed720 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	(_AT(pteval_t, 1) << 2)	 /* only for swp ptes */
>>
>> I think we can use bit 1 here.
>>
>>> @@ -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.
> 
> I'm a little worried that when we're dealing with huge mappings at the
> PMD level we might lose the ability to distinguish them from a pte-level
> mapping with this new flag set if we use bit 1. A similar issue to this
> was fixed a long time ago by 59911ca4325d ("ARM64: mm: Move PTE_PROT_NONE
> bit") when we used to use bit 1 for PTE_PROT_NONE.
> 
> Is something like:
> 
> 	pmd_to_swp_entry(swp_entry_to_pmd(pmd));

Note that __HAVE_ARCH_PTE_SWP_EXCLUSIVE currently only applies to actual
swap entries, not non-swap entries (migration, hwpoison, ...). So it
really only applies to PTEs -- PMDs are not applicable.

So the example you gave cannot possibly have that bit set. From what I
understand, it should be fine. But I have no real preference: I can also
just stick to the original patch, whatever you prefer.

Thanks!

-- 
Thanks,

David / dhildenb