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

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Date:   Fri, 5 Mar 2021 19:26:47 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     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 04.03.21 18:08, Dave Hansen wrote:
> 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.
> 

For this reason the s390x part by me (see patch description) was two 
separate patches. Maybe it also makes sense to split it up here.

-- 
Thanks,

David / dhildenb