Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 11/19/2015 09:18 AM, Martin Schwidefsky wrote:
> On Thu, 19 Nov 2015 00:49:58 +0100
> Dominik Dingel <dingel@linux.vnet.ibm.com> wrote:
> 
>> The userfaultfd does need FAULT_FLAG_ALLOW_RETRY to not return
>> VM_FAULT_SIGBUS.  So we improve the gmap code to handle one
>> VM_FAULT_RETRY.
>>
>> Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
>> ---
>>  arch/s390/mm/pgtable.c | 28 ++++++++++++++++++++++++----
>>  1 file changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
>> index 54ef3bc..8a0025d 100644
>> --- a/arch/s390/mm/pgtable.c
>> +++ b/arch/s390/mm/pgtable.c
>> @@ -577,15 +577,22 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
>>  	       unsigned int fault_flags)
>>  {
>>  	unsigned long vmaddr;
>> -	int rc;
>> +	int rc, fault;
>>
>> +	fault_flags |= FAULT_FLAG_ALLOW_RETRY;
>> +retry:
>>  	down_read(&gmap->mm->mmap_sem);
>>  	vmaddr = __gmap_translate(gmap, gaddr);
>>  	if (IS_ERR_VALUE(vmaddr)) {
>>  		rc = vmaddr;
>>  		goto out_up;
>>  	}
>> -	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
>> +	fault = fixup_user_fault(current, gmap->mm, vmaddr, fault_flags);
>> +	if (fault & VM_FAULT_RETRY) {
>> +		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> +		fault_flags |= FAULT_FLAG_TRIED;
>> +		goto retry;
>> +	} else if (fault) {
>>  		rc = -EFAULT;
>>  		goto out_up;
>>  	}
> 
> Me thinks that you want to add the retry code into fixup_user_fault itself.
> You basically have the same code around the three calls to fixup_user_fault.
> Yes, it will be a common code patch but I guess that it will be acceptable
> given userfaultfd as a reason.

That makes a lot of sense. In an earlier discussion (a followup of Jasons
mm: Loosen MADV_NOHUGEPAGE to enable Qemu postcopy on s390) patch.

Andrea suggested the following:

It's probably better to add a fixup_user_fault_unlocked that will work
like get_user_pages_unlocked. I.e. leaves the details of the mmap_sem
locking internally to the function, and will handle VM_FAULT_RETRY
automatically by re-taking the mmap_sem and repeating the
fixup_user_fault after updating the FAULT_FLAG_ALLOW_RETRY to
FAULT_FLAG_TRIED.

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