[RFC PATCH] powerpc/book3s/hash: Drop _PAGE_PRIVILEGED from PAGE_NONE

[Aneesh Kumar K V]

On 11/13/23 3:46 PM, Nicholas Piggin wrote:
> On Thu Nov 2, 2023 at 11:23 PM AEST, Aneesh Kumar K.V wrote:
>> There used to be a dependency on _PAGE_PRIVILEGED with pte_savedwrite.
>> But that got dropped by
>> commit 6a56ccbcf6c6 ("mm/autonuma: use can_change_(pte|pmd)_writable() to replace savedwrite")
>>
>> With this change numa fault pte (pte_protnone()) gets mapped as regular user pte
>> with RWX cleared (no-access).
> 
> You mean "that" above change (not *this* change), right?
> 

With the change in this patch numa fault pte will not have _PAGE_PRIVILEGED set because 
PAGE_NONE now maps to just _PAGE_BASE


>> This also remove pte_user() from
>> book3s/64.
> 
> Nice cleanup. That was an annoying hack.
> 
>> pte_access_permitted() now checks for _PAGE_EXEC because we now support
>> EXECONLY mappings.
> 
> AFAIKS pte_exec() is not required, GUP is really only for read or
> write access. It should be a separate patch if you think it's needed.
> 

I have a v2 dropping that based on https://lore.kernel.org/linux-mm/87bkc1oe8c.fsf@linux.ibm.com 
I kept pte_user with pte_access_permitted being the only user. I can open code that
if needed. 

>>
>> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar at linux.ibm.com>
>> ---
>>  arch/powerpc/include/asm/book3s/64/pgtable.h | 23 +++++---------------
>>  arch/powerpc/mm/book3s64/hash_utils.c        | 17 +++++++++++++++
>>  2 files changed, 23 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index cb77eddca54b..7c7de7b56df0 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -17,12 +17,6 @@
>>  #define _PAGE_EXEC		0x00001 /* execute permission */
>>  #define _PAGE_WRITE		0x00002 /* write access allowed */
>>  #define _PAGE_READ		0x00004	/* read access allowed */
>> -#define _PAGE_NA		_PAGE_PRIVILEGED
>> -#define _PAGE_NAX		_PAGE_EXEC
>> -#define _PAGE_RO		_PAGE_READ
>> -#define _PAGE_ROX		(_PAGE_READ | _PAGE_EXEC)
>> -#define _PAGE_RW		(_PAGE_READ | _PAGE_WRITE)
>> -#define _PAGE_RWX		(_PAGE_READ | _PAGE_WRITE | _PAGE_EXEC)
>>  #define _PAGE_PRIVILEGED	0x00008 /* kernel access only */
>>  #define _PAGE_SAO		0x00010 /* Strong access order */
>>  #define _PAGE_NON_IDEMPOTENT	0x00020 /* non idempotent memory */
> 
> Did you leave PAGE_NONE as _PAGE_BASE | _PAGE_PRIVILEGED below?
> Shouldn't that be changed too? Then this patch is not only hash
> but also radix.
> 

A recent patch moved PAGE_NONE to pgtable-mask.h 
a5a08dc90f4513d1a78582ec24b687fad01cc843

> Why is the hash change required? Previously PAGE_NONE relied on
> privileged bit to prevent access, now you need to handle a PTE
> without that bit? In that case could that be patch 1, then the
> rest patch 2?
> 

Looking at older kernel, I guess check_pte_access used _PAGE_PRIVILEGED 
as a way to prevent access to PAGE_NONE ptes. We now depend on
_PAGE_READ

> __pte_flags_need_flush() should be updated after this too,
> basically revert commit 1abce0580b894.
> 

Will update the patch to include the revert.

>> @@ -119,9 +113,9 @@
>>  /*
>>   * user access blocked by key
>>   */
>> -#define _PAGE_KERNEL_RW		(_PAGE_PRIVILEGED | _PAGE_RW | _PAGE_DIRTY)
>>  #define _PAGE_KERNEL_RO		 (_PAGE_PRIVILEGED | _PAGE_READ)
>>  #define _PAGE_KERNEL_ROX	 (_PAGE_PRIVILEGED | _PAGE_READ | _PAGE_EXEC)
>> +#define _PAGE_KERNEL_RW		(_PAGE_PRIVILEGED | _PAGE_RW | _PAGE_DIRTY)
>>  #define _PAGE_KERNEL_RWX	(_PAGE_PRIVILEGED | _PAGE_DIRTY | _PAGE_RW | _PAGE_EXEC)
>>  /*
>>   * _PAGE_CHG_MASK masks of bits that are to be preserved across
> 
> No need to reorder defines.
> 

ok


> Thanks,
> Nick
> 
>> @@ -523,19 +517,14 @@ static inline bool arch_pte_access_permitted(u64 pte, bool write, bool execute)
>>  }
>>  #endif /* CONFIG_PPC_MEM_KEYS */
>>  
>> -static inline bool pte_user(pte_t pte)
>> -{
>> -	return !(pte_raw(pte) & cpu_to_be64(_PAGE_PRIVILEGED));
>> -}
>> -
>>  #define pte_access_permitted pte_access_permitted
>>  static inline bool pte_access_permitted(pte_t pte, bool write)
>>  {
>> -	/*
>> -	 * _PAGE_READ is needed for any access and will be
>> -	 * cleared for PROT_NONE
>> -	 */
>> -	if (!pte_present(pte) || !pte_user(pte) || !pte_read(pte))
>> +
>> +	if (!pte_present(pte))
>> +		return false;
>> +
>> +	if (!(pte_read(pte) || pte_exec(pte)))
>>  		return false;
>>  
>>  	if (write && !pte_write(pte))
>> diff --git a/arch/powerpc/mm/book3s64/hash_utils.c b/arch/powerpc/mm/book3s64/hash_utils.c
>> index ad2afa08e62e..b2eda22195f0 100644
>> --- a/arch/powerpc/mm/book3s64/hash_utils.c
>> +++ b/arch/powerpc/mm/book3s64/hash_utils.c
>> @@ -310,9 +310,26 @@ unsigned long htab_convert_pte_flags(unsigned long pteflags, unsigned long flags
>>  			else
>>  				rflags |= 0x3;
>>  		}
>> +		WARN_ON(!(pteflags & _PAGE_RWX));
>>  	} else {
>>  		if (pteflags & _PAGE_RWX)
>>  			rflags |= 0x2;
>> +		else {
>> +			/*
>> +			 * PAGE_NONE will get mapped to 0b110 (slb key 1 no access)
>> +			 * We picked 0b110 instead of 0b000 so that slb key 0 will
>> +			 * get only read only access for the same rflags.
>> +			 */
>> +			if (mmu_has_feature(MMU_FTR_KERNEL_RO))
>> +				rflags |= (HPTE_R_PP0 | 0x2);
>> +			/*
>> +			 * rflags = HPTE_R_N
>> +			 * Without KERNEL_RO feature this will result in slb
>> +			 * key 0 with read/write. But ISA only supports that.
>> +			 * There is no key 1 no-access and key 0 read-only
>> +			 * pp bit support.
>> +			 */
>> +		}
>>  		if (!((pteflags & _PAGE_WRITE) && (pteflags & _PAGE_DIRTY)))
>>  			rflags |= 0x1;
>>  	}
> 

V2 is also dropping the above change, because we will never have hash table entries inserted. 

This is added to commit message.

    Hash fault handling code did get some WARN_ON added because those
    functions are not expected to get called with _PAGE_READ cleared.
    commit 18061c17c8ec ("powerpc/mm: Update PROTFAULT handling in the page fault path")
    explains the details.



-aneesh