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

[Peter Xu]

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.

Thanks!

-- 
Peter Xu