Re: [RFC 08/14] s390/mm: Make gmap_read_table EDAT1 compatible

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On 16.10.18 10:37, David Hildenbrand wrote:
> On 19/09/2018 10:47, Janosch Frank wrote:
>> For the upcoming support of VSIE guests on huge page backed hosts, we
>> need to be able to read from large segments.
>>
>> Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
>> ---
>>  arch/s390/mm/gmap.c | 43 ++++++++++++++++++++++++++-----------------
>>  1 file changed, 26 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
>> index 26cc6ce19afb..ba0425f1c2c0 100644
>> --- a/arch/s390/mm/gmap.c
>> +++ b/arch/s390/mm/gmap.c
>> @@ -1274,35 +1274,44 @@ EXPORT_SYMBOL_GPL(gmap_mprotect_notify);
>>  int gmap_read_table(struct gmap *gmap, unsigned long gaddr, unsigned long *val)
>>  {
>>  	unsigned long address, vmaddr;
>> -	spinlock_t *ptl;
>> +	spinlock_t *ptl_pmd = NULL, *ptl_pte = NULL;
>> +	pmd_t *pmdp;
>>  	pte_t *ptep, pte;
>>  	int rc;
>>  
>> -	if (gmap_is_shadow(gmap))
>> -		return -EINVAL;
>> +	BUG_ON(gmap_is_shadow(gmap));
> 
> Why this change? This is documented behavior (and this is an exported
> function - I think we should keep performing validity checks on exported
> functions).

Sure, I can drop the change.

> 
>>  
>>  	while (1) {
>>  		rc = -EAGAIN;
>> -		ptep = gmap_pte_op_walk(gmap, gaddr, &ptl);
>> -		if (ptep) {
>> -			pte = *ptep;
>> -			if (pte_present(pte) && (pte_val(pte) & _PAGE_READ)) {
>> -				address = pte_val(pte) & PAGE_MASK;
>> -				address += gaddr & ~PAGE_MASK;
>> +		vmaddr = __gmap_translate(gmap, gaddr);
>> +		if (IS_ERR_VALUE(vmaddr))
>> +			return vmaddr;
>> +		pmdp = gmap_pmd_op_walk(gmap, gaddr, vmaddr, &ptl_pmd);
>> +		if (pmdp && !(pmd_val(*pmdp) & _SEGMENT_ENTRY_INVALID)) {
>> +			if (!pmd_large(*pmdp)) {
>> +				ptep = gmap_pte_from_pmd(gmap, pmdp, vmaddr, &ptl_pte);
>> +				if (ptep) {
>> +					pte = *ptep;
>> +					if (pte_present(pte) && (pte_val(pte) & _PAGE_READ)) {
>> +						address = pte_val(pte) & PAGE_MASK;
>> +						address += gaddr & ~PAGE_MASK;
>> +						*val = *(unsigned long *) address;
>> +						pte_val(*ptep) |= _PAGE_YOUNG;
>> +						/* Do *NOT* clear the _PAGE_INVALID bit! */
>> +						rc = 0;
>> +					}
>> +				}
>> +				gmap_pte_op_end(ptl_pte);
> 
> I'm confused that we have a gmap_pte_op_end() followed by a
> gmap_pmd_op_end() although we never started a gmap_pte_op_walk() ... I
> assume this is due to gmap_pte_from_pmd() ? We should find better names
> for these functions otherwise this is pure magic.
> 
> e.g. gmap_pte_op_walk_pmd() instead of gmap_pte_from_pmd()

Hrm, in my opinion pte_from_pmd is very specific, although it lacks the
op part. How about gmap_pte_op_map, that would be closer to the
pte_alloc/offset_map from the kernel side?

> 
> ... and shouldn't "gmap_pte_op_end(ptl_pte)" be inside of the "if(ptep)" ?

It doesn't matter as we check the ptl for NULL in op_end functions.
I have that scheme everywhere where it's nicer to read, like in the gmap
protection functions.

I'll have a look if I can make that consistent either way.


> 
>> +			} else {
>> +				address = pmd_val(*pmdp) & HPAGE_MASK;
>> +				address += gaddr & ~HPAGE_MASK;
>>  				*val = *(unsigned long *) address;
>> -				pte_val(*ptep) |= _PAGE_YOUNG;
>> -				/* Do *NOT* clear the _PAGE_INVALID bit! */
>>  				rc = 0;
>>  			}
>> -			gmap_pte_op_end(ptl);
>> +			gmap_pmd_op_end(ptl_pmd);
>>  		}
>>  		if (!rc)
>>  			break;
>> -		vmaddr = __gmap_translate(gmap, gaddr);
>> -		if (IS_ERR_VALUE(vmaddr)) {
>> -			rc = vmaddr;
>> -			break;
>> -		}
>>  		rc = gmap_fixup(gmap, gaddr, vmaddr, PROT_READ);
>>  		if (rc)
>>  			break;
>>
> 
> 


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