Re: [PATCH v4 01/26] mm: Do page fault accounting in handle_mm_fault

[Peter Xu]

Hi, David,

On Tue, Jun 30, 2020 at 02:05:24PM -0700, David Rientjes wrote:
> On Tue, 30 Jun 2020, Peter Xu wrote:
> 
> > @@ -4408,6 +4440,34 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> >  			mem_cgroup_oom_synchronize(false);
> >  	}
> >  
> > +	if (ret & (VM_FAULT_RETRY | VM_FAULT_ERROR))
> > +		return ret;
> > +
> > +	/*
> > +	 * Do accounting in the common code, to avoid unnecessary
> > +	 * architecture differences or duplicated code.
> > +	 *
> > +	 * We arbitrarily make the rules be:
> > +	 *
> > +	 *  - Unsuccessful faults do not count (e.g. when the address wasn't
> > +	 *    valid). That includes arch_vma_access_permitted() failing above.
> > +	 *
> > +	 *    So this is expressly not a "this many hardware page faults"
> > +	 *    counter. Use the hw profiling for that.
> > +	 *
> > +	 *  - Incomplete faults do not count (e.g. RETRY).  They will only
> > +	 *    count once completed.
> > +	 *
> > +	 *  - The fault counts as a "major" fault when the final successful
> > +	 *    fault is VM_FAULT_MAJOR, or if it was a retry (which implies that
> > +	 *    we couldn't handle it immediately previously).
> > +	 *
> > +	 *  - If the fault is done for GUP, regs will be NULL and no accounting
> > +	 *    will be done.
> > +	 */
> > +	mm_account_fault(regs, address, (ret & VM_FAULT_MAJOR) ||
> > +			 (flags & FAULT_FLAG_TRIED));
> > +
> >  	return ret;
> >  }
> >  EXPORT_SYMBOL_GPL(handle_mm_fault);
> 
> Just a nit, likely not important: I wonder if it would be cleaner to pass 
> the vm_fault_t into mm_account_fault() and then do the VM_FAULT_RETRY and
> VM_FAULT_ERROR checks there as well as putting the comment about how 
> accounting is handled in that function.  Your comment is great.

Yes that seems to be cleaner so handle_mm_fault is shorter (btw, I "stole" the
comment block majorly from Linus :).

But this change will also need to touch patch 25 again or it won't apply
cleanly.  So I think I'll see whether Andrew would like me to repost the whole
series then I'll adopt the change when I repost, or another alternative is
maybe we can also do that on top, depending on whether there's further
comments..

Thanks!

-- 
Peter Xu