Re: [PATCH] um: asm/page.h: zero out a pte's high value in set_pte_val()

[Nicolai Stange]

Richard Weinberger <richard@xxxxxx> writes:

> Am 29.01.2016 um 00:56 schrieb Nicolai Stange:
>> Commit 16da306849d0 ("um: kill pfn_t")
>> introduced a compile warning for defconfig:
>> 
>>   arch/um/kernel/skas/mmu.c:38:206: warning: right shift count >= width of type
>>                                     [-Wshift-count-overflow]
>> 
>> Aforementioned patch changes the definition of the phys_to_pfn() macro from
>> 
>>   ((pfn_t) ((p) >> PAGE_SHIFT))
>> 
>> to
>> 
>>   ((p) >> PAGE_SHIFT)
>> 
>> This effectively changes the phys_to_pfn() expansion's type from
>> unsigned long long to unsigned long.
>> 
>> Through the callchain init_stub_pte()->mk_pte(), the expansion of
>> phys_to_pfn() is (indirectly) fed into the 'phys' argument of the
>> pte_set_val(pte, phys, prot) macro, eventually leading to
>> 
>>   (pte).pte_high = (phys) >> 32;
>> 
>> This results in the warning from above.
>> 
>> Since UML only deals with 32 bit addresses, the upper 32 bits from 'phys'
>> used to be zero anyway.
>> 
>> Zero out the pte value's high part in pte_set_val() in order to get rid
>> of the offending shift.
>> 
>> Fixes: 16da306849d0 ("um: kill pfn_t")
>> Signed-off-by: Nicolai Stange <nicstange@xxxxxxxxx>
>> ---
>>  arch/um/include/asm/page.h | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/arch/um/include/asm/page.h b/arch/um/include/asm/page.h
>> index e13d41c..61e235f 100644
>> --- a/arch/um/include/asm/page.h
>> +++ b/arch/um/include/asm/page.h
>> @@ -46,8 +46,8 @@ typedef struct { unsigned long pgd; } pgd_t;
>>  			      smp_wmb(); \
>>  			      (to).pte_low = (from).pte_low; })
>>  #define pte_is_zero(pte) (!((pte).pte_low & ~_PAGE_NEWPAGE) && !(pte).pte_high)
>> -#define pte_set_val(pte, phys, prot) \
>> -	({ (pte).pte_high = (phys) >> 32; \
>> +#define pte_set_val(pte, phys, prot)				\
>> +	({ (pte).pte_high = 0;					\
>>  	   (pte).pte_low = (phys) | pgprot_val(prot); })
>
> I think we can completely kill .pte_high.
>

I did a quick test with ->pte_high purged and this doesn't introduce any
new warnings.

Booting w/o a rootfs works up to mount_root.


Note that an implication of getting rid of ->pte_high would be that the
type of pte_val() would get changed from unsigned long long to unsigned
long. However, outside of arch/um, pte_val() is only used here:
- drivers/gpu/drm/drm_vm.c
- include/trace/events/xen.h
- mm/gup.c
- mm/memory.c
All these uses and the ones in arch/um itself look compatible with this
change (if relevant at all for UML).


I'll post a follow up patch for this tomorrow.

Question 1: now that ->pte_high will be gone, do you want to have
            ->pte_low renamed to e.g. ->pte_val?

Question 2: what is the smp_wmb() in pte_copy() paired with/good for?


Thank you!

Nicolai