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Date: Thu, 28 Sep 2017 11:39:55 +0200
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCHv7 01/19] mm/sparsemem: Allocate mem_section at runtime
 for SPARSEMEM_EXTREME
Message-ID: <20170928093955.lhwzkdqsshrhobnl@gmail.com>
References: <20170918105553.27914-1-kirill.shutemov@linux.intel.com>
 <20170918105553.27914-2-kirill.shutemov@linux.intel.com>
 <20170928080711.3msbrmwluqwfhjkg@gmail.com>
 <20170928090850.oed4ls6ojpbjpkcp@node.shutemov.name>
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Ingo Molnar <mingo@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>, Andrew Morton <akpm@linux-foundation.org>, Andy Lutomirski <luto@amacapital.net>, Cyrill Gorcunov <gorcunov@openvz.org>, Borislav Petkov <bp@suse.de>, linux-mm@kvack.org, linux-kernel@vger.kernel.org


* Kirill A. Shutemov <kirill@shutemov.name> wrote:

> On Thu, Sep 28, 2017 at 10:07:11AM +0200, Ingo Molnar wrote:
> > 
> > * Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:
> > 
> > > Size of mem_section array depends on size of physical address space.
> > > 
> > > In preparation for boot-time switching between paging modes on x86-64
> > > we need to make allocation of mem_section dynamic.
> > > 
> > > The patch allocates the array on the first call to
> > > sparse_memory_present_with_active_regions().
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > 
> > What is the size of the allocations here, in bytes, for the two main variants?
> 
> For 4-level paging it's 32k. For 5-level paging it's 2M.

Ok, please put this information into the changelog, because it's relevant: we 
don't want to waste 2MB of RAM on the majority of systems that boot a distro 
kernel with 5-level paging enabled.

Thanks,

	Ingo

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