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    SubjectRe: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault handler retries
    On Tue, Apr 18, 2023 at 7:25 AM Matthew Wilcox <willy@infradead.org> wrote:
    >
    > On Mon, Apr 17, 2023 at 04:17:45PM -0700, Suren Baghdasaryan wrote:
    > > On Mon, Apr 17, 2023 at 3:52 PM Matthew Wilcox <willy@infradead.org> wrote:
    > > >
    > > > 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.
    > >
    > > IIUC, the question was about the differences in vm vs perf accounting
    > > for errors, not the difference between ERROR and RETRY cases. Matthew,
    > > are you answering the right question or did I misunderstand your
    > > answer?
    >
    > Maybe I'm misunderstanding what you're proposing. I thought the
    > proposal was to make neither ERROR nor RETRY increment the counters,
    > but if the proposal is to make ERROR increment the perf counters
    > instead, then that's cool with me.

    Oh, I think now I understand your answer. You were not highlighting
    the difference between the who but objecting to the proposal of not
    counting both ERROR and RETRY. Am I on the same page now?

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