linux-kernel - Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

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Message-ID: <CAK1f24=wK9Gpt71NtVCyM1RRQF0MDmhm4MViVj_gPx_+ywks7w@mail.gmail.com>
Date: Fri, 5 Jul 2024 19:19:15 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org, dj456119@...il.com, 
	ryan.roberts@....com, shy828301@...il.com, ziy@...dia.com, 
	libang.li@...group.com, baolin.wang@...ux.alibaba.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Mingzhe Yang <mingzhe.yang@...com>
Subject: Re: [PATCH v3 1/2] mm: add per-order mTHP split counters

On Fri, Jul 5, 2024 at 6:56 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 05.07.24 12:48, Lance Yang wrote:
> > Hi David and Barry,
> >
> > Thanks a lot for paying attention!
> >
> > On Fri, Jul 5, 2024 at 6:14 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 05.07.24 12:12, Barry Song wrote:
> >>> On Fri, Jul 5, 2024 at 9:08 PM David Hildenbrand <david@...hat.com> wrote:
> >>>>
> >>>>> @@ -3253,8 +3259,9 @@ int split_huge_page_to_list_to_order(struct page *page, struct list_head *list,
> >>>>>                 i_mmap_unlock_read(mapping);
> >>>>>     out:
> >>>>>         xas_destroy(&xas);
> >>>>> -     if (is_thp)
> >>>>> +     if (order >= HPAGE_PMD_ORDER)
> >>>>
> >>>> We likely should be using "== HPAGE_PMD_ORDER" here, to be safe for the
> >>>> future.
> >>>
> >>> I feel this might need to be separate since all other places are using
> >>> folio_test_pmd_mappable() ?
> >>
> >> Likely, but as you are moving away from this ... this counter here does
> >> and will always only care about HPAGE_PMD_ORDER.
> >
> > I appreciate the different opinions on whether we should use
> > ">= HPAGE_PMD_ORDER" or "==" for this check.
> >
> > In this context, let's leave it as is and stay consistent with
> > folio_test_pmd_mappable() by using ">= HPAGE_PMD_ORDER",
> > what do you think?
>
> I don't think it's a good idea to add more wrong code that is even
> harder to grep (folio_test_pmd_mappable would give you candidates that
> might need attention). But I don't care too much. Maybe someone here can
> volunteer to clean up these instances to make sure we check PMD-size and
> not PMD-mappable for these counters that are for PMD-sized folios only,
> even in the future with larger folios?

Thanks for clarifying! Yes, agreed. We should ensure we check PMD-size,
not PMD-mappable here, especially as we consider large folios in the future.

So, let's use "== HPAGE_PMD_ORDER" here ;)

Thanks,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>