linux-kernel - Re: [PATCH 01/26] mm: Do page fault accounting in handle_mm

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Message-ID: <20200624204903.097a5a58@thinkpad>
Date:   Wed, 24 Jun 2020 20:49:03 +0200
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 01/26] mm: Do page fault accounting in handle_mm_fault

On Fri, 19 Jun 2020 12:05:13 -0400
Peter Xu <peterx@...hat.com> wrote:

[...]

> @@ -4393,6 +4425,38 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>  			mem_cgroup_oom_synchronize(false);
>  	}
> 
> +	if (ret & VM_FAULT_RETRY)
> +		return ret;

I'm wondering if this also needs a check and exit for VM_FAULT_ERROR.
In arch code (s390 and all others I briefly checked), the accounting
was skipped for VM_FAULT_ERROR case.

> +
> +	/*
> +	 * Do accounting in the common code, to avoid unnecessary
> +	 * architecture differences or duplicated code.
> +	 *
> +	 * We arbitrarily make the rules be:
> +	 *
> +	 *  - faults that never even got here (because the address
> +	 *    wasn't valid). That includes arch_vma_access_permitted()

Missing "do not count" at the end of the first sentence?

> +	 *    failing above.
> +	 *
> +	 *    So this is expressly not a "this many hardware page
> +	 *    faults" counter. Use the hw profiling for that.
> +	 *
> +	 *  - incomplete faults (ie RETRY) do not count (see above).
> +	 *    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 wil be NULL and

wil -> will

Regards,
Gerald