linux-kernel - Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault handler retries

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Message-ID: <ZD3Nk0u+nxOT4snZ@casper.infradead.org>
Date:   Mon, 17 Apr 2023 23:52:03 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        hannes@...xchg.org, mhocko@...e.com, josef@...icpanda.com,
        jack@...e.cz, ldufour@...ux.ibm.com, laurent.dufour@...ibm.com,
        michel@...pinasse.org, liam.howlett@...cle.com, jglisse@...gle.com,
        vbabka@...e.cz, minchan@...gle.com, dave@...olabs.net,
        punit.agrawal@...edance.com, lstoakes@...il.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page
 fault handler retries

On Mon, Apr 17, 2023 at 03:40:33PM -0400, Peter Xu wrote:
> >  	/*
> > -	 * We don't do accounting for some specific faults:
> > -	 *
> > -	 * - Unsuccessful faults (e.g. when the address wasn't valid).  That
> > -	 *   includes arch_vma_access_permitted() failing before reaching here.
> > -	 *   So this is not a "this many hardware page faults" counter.  We
> > -	 *   should use the hw profiling for that.
> > -	 *
> > -	 * - Incomplete faults (VM_FAULT_RETRY).  They will only be counted
> > -	 *   once they're completed.
> > +	 * Do not account for incomplete faults (VM_FAULT_RETRY). They will be
> > +	 * counted upon completion.
> >  	 */
> > -	if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY))
> > +	if (ret & VM_FAULT_RETRY)
> > +		return;
> > +
> > +	/* Register both successful and failed faults in PGFAULT counters. */
> > +	count_vm_event(PGFAULT);
> > +	count_memcg_event_mm(mm, PGFAULT);
> 
> Is there reason on why vm events accountings need to be explicitly
> different from perf events right below on handling ERROR?

I think so.  ERROR is quite different from RETRY.  If we are, for
example, handling a SIGSEGV (perhaps a GC language?) that should be
accounted.  If we can't handle a page fault right now, and need to
retry within the kernel, that should not be accounted.