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

[Dominik Dingel <dingel@linux.vnet.ibm.com>]

On Thu, 19 Nov 2015 09:25:24 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 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.

I know, I saw his mail. But within the gmap code we need to take the mmap_sem before calling fixup_user_fault as well as holding it for later on like __gmap_link.

We could introduce a new wrapper arround fixup_user_fault, like:
fixup_user_fault_retry, which would take care of the retry logic, but does not encapsulate the complete mmap_sem logic.

@Kirill would that be acceptable for you as well?

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