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From:   "Huang, Ying" <ying.huang@intel.com>
To:     Yu Zhao <yuzhao@google.com>
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Subject: Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork
References: <20220309021230.721028-1-yuzhao@google.com>
        <20220309021230.721028-6-yuzhao@google.com>
        <875yoh552i.fsf@yhuang6-desk2.ccr.corp.intel.com>
        <CAOUHufYPSesiePfxaV=y9Vne5cb+Y_vQtJyQ1NiO1CFus=8WOA@mail.gmail.com>
Date:   Tue, 15 Mar 2022 08:34:18 +0800
In-Reply-To: <CAOUHufYPSesiePfxaV=y9Vne5cb+Y_vQtJyQ1NiO1CFus=8WOA@mail.gmail.com>
        (Yu Zhao's message of "Mon, 14 Mar 2022 03:30:31 -0600")
Message-ID: <87mths3vg5.fsf@yhuang6-desk2.ccr.corp.intel.com>
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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.

#define LAST__PID_SHIFT 8
#define LAST__PID_MASK  ((1 << LAST__PID_SHIFT)-1)

#define LAST__CPU_SHIFT NR_CPUS_BITS
#define LAST__CPU_MASK  ((1 << LAST__CPU_SHIFT)-1)

#define LAST_CPUPID_SHIFT (LAST__PID_SHIFT+LAST__CPU_SHIFT)

Best Regards,
Huang, Ying

[snip]