linux-kernel - Re: [PATCH v4 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges

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Message-ID: <20210308184840.GC25767@linux>
Date:   Mon, 8 Mar 2021 19:48:40 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges

On Thu, Mar 04, 2021 at 10:42:59AM -0800, Dave Hansen wrote:
> On 3/1/21 12:32 AM, Oscar Salvador wrote:
> > We never get to allocate 1GB pages when mapping the vmemmap range.
> > Drop the dead code both for the aligned and unaligned cases and leave
> > only the direct map handling.
> 
> Could you elaborate a bit on why 1GB pages are never used?  It is just
> unlikely to have a 64GB contiguous area of memory that needs 1GB of
> contiguous vmemmap?  Or, does the fact that sections are smaller than
> 64GB keeps this from happening?

AFAIK, the biggest we populate vmemmap pages with is 2MB, plus the fact
that as you pointed out, memory sections on x86_64 are 128M, which is
way smaller than what would require to allocate a 1GB for vmemmap pages.

Am I missing something?


-- 
Oscar Salvador
SUSE L3