linux-kernel - Re: [PATCH v2 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges

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Date:   Wed, 3 Feb 2021 15:12:05 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        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 v2 2/3] x86/vmemmap: Drop handling of 1GB vmemmap ranges

On 03.02.21 15:10, Oscar Salvador wrote:
> On Wed, Feb 03, 2021 at 02:33:56PM +0100, David Hildenbrand wrote:
>> One problem I see with existing code / this change making more obvious is
>> that when trying to remove in other granularity than we added (e.g., unplug
>> a 128MB DIMM avaialble during boot), we remove the direct map of unrelated
>> DIMMs.
> 
> So, let me see if I understand your concern.
> 
> We have a range that was mapped with 1GB page, and we try to remove
> a 128MB chunk from it.
> Yes, in that case we would clear the pud, and that is bad, so we should
> keep the PAGE_ALIGNED checks.
> 
> Now, let us assume that scenario.
> If you have a 1GB mapped range and you remove it in smaller chunks bit by bit
> (e.g: 128M), the direct mapping of that range will never be cleared unless

No, that's exactly what's happening. Good thing is that it barely ever 
happens, so I assume leaving behind some direct mapping / page tables is 
not that bad.

-- 
Thanks,

David / dhildenb