Re: Possible duplicate page fault accounting on some archs after commit 4064b9827063

[Peter Xu]

On Wed, Jun 10, 2020 at 12:50:23PM -0400, Peter Xu wrote:
> On Wed, Jun 10, 2020 at 05:48:11PM +0200, Gerald Schaefer wrote:
> > Hi,
> 
> Hi, Gerald,
> 
> > 
> > Some architectures have their page fault accounting code inside the fault
> > retry loop, and rely on only going through that code once. Before commit
> > 4064b9827063 ("mm: allow VM_FAULT_RETRY for multiple times"), that was
> > ensured by testing for and clearing FAULT_FLAG_ALLOW_RETRY.
> > 
> > That commit had to remove the clearing of FAULT_FLAG_ALLOW_RETRY for all
> > architectures, and introduced a subtle change to page fault accounting
> > logic in the affected archs. It is now possible to go through the retry
> > loop multiple times, and the affected archs would then account multiple
> > page faults instead of just one.
> > 
> > This was found by coincidence in s390 code, and a quick check showed that
> > there are quite a lot of other architectures that seem to be affected in a
> > similar way. I'm preparing a fix for s390, by moving the accounting behind
> > the retry loop, similar to x86. It is not completely straight-forward, so
> > I leave the fix for other archs to the respective maintainers.
> 
> Sorry for not noticing this before.  The accounting part should definitely be
> put at least into a check against fault_flag_allow_retry_first() to mimic what
> was done before.  And I agree it would be even better to put it after the retry
> logic, so if any of the page faults gets a major fault, it'll be accounted as a
> major fault which makes more sense to me, just like what x86 is doing now with:
> 
> 	major |= fault & VM_FAULT_MAJOR;
> 
> I'm not sure what's the preference of the arch maintainers, just let me know if
> it's preferred to use a single series to address this issue for all affected
> archs (or the archs besides s390), then I'll do.

To make sure this won't fall through the cracks... I'll give it a shot with a
single series to address this issue for all archs.  Although it might not be
easy to do accounting directly in handle_mm_fault(), it might be still a chance
to introduce a helper so the accounting can be done in general code.

Thanks,

-- 
Peter Xu