Richard Weinberger <rich...@nod.at> 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 <nicsta...@gmail.com>
>> ---
>> 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
