On 11/19/2015 09:18 AM, Martin Schwidefsky wrote:
> On Thu, 19 Nov 2015 00:49:58 +0100
> Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx>
>> ---
>> 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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