linux-kernel - Re: [PATCH v4 3/3] x86/vmemmap: Handle unpopulated sub-pmd ranges

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Date:   Thu, 4 Mar 2021 09:08:37 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     David Hildenbrand <david@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] x86/vmemmap: Handle unpopulated sub-pmd ranges

On 3/4/21 9:02 AM, Dave Hansen wrote:
>> +#define PAGE_UNUSED 0xFD
>> +/*
>> + * The unused vmemmap range, which was not yet memset(PAGE_UNUSED) ranges
>> + * from unused_pmd_start to next PMD_SIZE boundary.
>> + */
>> +static unsigned long unused_pmd_start __meminitdata;
> This whole 'unused_pmd_start' thing was unmentioned in the changelog.

One tiny suggestion: *Sometimes* for these optimizations, it's easiest
to write the code up without it in one patch, then add the optimization
in the next patch.

It makes it 100% clear what is part of the "core" algorithm and what is
pure optimization.

I don't know if it will work here, but it might be worth taking a look.