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

From: David Rientjes
Date: Tue Jun 30 2020 - 17:05:29 EST


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.