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From:   Yu Zhao <yuzhao@google.com>
Date:   Mon, 14 Mar 2022 18:50:08 -0600
Message-ID: <CAOUHufY8_f353GivFy=_2g=DHmQJcBHNzbabhyGhMnoBbzL8yQ@mail.gmail.com>
Subject: Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork
To:     "Huang, Ying" <ying.huang@intel.com>
Cc:     kernel@lists.fedoraproject.org, kernel-team@lists.ubuntu.com,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <ak@linux.intel.com>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
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On Mon, Mar 14, 2022 at 6:34 PM Huang, Ying <ying.huang@intel.com> wrote:
>
> Yu Zhao <yuzhao@google.com> writes:
>
> > On Mon, Mar 14, 2022 at 2:09 AM Huang, Ying <ying.huang@intel.com> wrote:
> >>
> >> Hi, Yu,
> >>
> >> Yu Zhao <yuzhao@google.com> writes:
> >> > diff --git a/mm/Kconfig b/mm/Kconfig
> >> > index 3326ee3903f3..747ab1690bcf 100644
> >> > --- a/mm/Kconfig
> >> > +++ b/mm/Kconfig
> >> > @@ -892,6 +892,16 @@ config ANON_VMA_NAME
> >> >         area from being merged with adjacent virtual memory areas due to the
> >> >         difference in their name.
> >> >
> >> > +# the multi-gen LRU {
> >> > +config LRU_GEN
> >> > +     bool "Multi-Gen LRU"
> >> > +     depends on MMU
> >> > +     # the following options can use up the spare bits in page flags
> >> > +     depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP)
> >>
> >> LRU_GEN depends on !MAXSMP.  So, What is the maximum NR_CPUS supported
> >> by LRU_GEN?
> >
> > LRU_GEN doesn't really care about NR_CPUS. IOW, it doesn't impose a
> > max number. The dependency is with NODES_SHIFT selected by MAXSMP:
> >     default "10" if MAXSMP
> > This combined with LAST_CPUPID_SHIFT can exhaust the spare bits in page flags.
>
> From the following code snippets from page-flags-layout.h,
> LAST_CPUPID_SHIFT is related to NR_CPUS instead of NODES_SHIFT.

It is. But LAST_CPUPID_NOT_IN_PAGE_FLAGS should always work but
NODE_NOT_IN_PAGE_FLAGS doesn't.