linux-kernel - Re: [PATCHv2 5/7] x86/mm: Reserve unaccepted memory bitmap

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Date:   Sat, 15 Jan 2022 20:46:57 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 5/7] x86/mm: Reserve unaccepted memory bitmap

On Wed, Jan 12, 2022 at 11:53:42AM -0800, Dave Hansen wrote:
> On 1/12/22 11:43 AM, Kirill A. Shutemov wrote:
> > On Tue, Jan 11, 2022 at 11:10:40AM -0800, Dave Hansen wrote:
> >> On 1/11/22 03:33, Kirill A. Shutemov wrote:
> >>
> >>> +	/* Mark unaccepted memory bitmap reserved */
> >>> +	if (boot_params.unaccepted_memory) {
> >>> +		unsigned long size;
> >>> +
> >>> +		/* One bit per 2MB */
> >>> +		size = DIV_ROUND_UP(e820__end_of_ram_pfn() * PAGE_SIZE,
> >>> +				    PMD_SIZE * BITS_PER_BYTE);
> >>> +		memblock_reserve(boot_params.unaccepted_memory, size);
> >>> +	}
> >>
> >> Is it OK that the size of the bitmap is inferred from
> >> e820__end_of_ram_pfn()?  Is this OK in the presence of mem= and other things
> >> that muck with the e820?
> > 
> > Good question. I think we are fine. If kernel is not able to allocate
> > memory from a part of physical address space we don't need the bitmap for
> > it either.
> 
> That's a good point.  If the e820 range does a one-way shrink it's
> probably fine.  The only problem would be if the bitmap had space for
> for stuff past e820__end_of_ram_pfn() *and* it later needed to be accepted.

It's unlikely, but e820 can grow because of EFI and because of memmap=.
To be completely on the safe side, the unaccepted bitmap should be reserved
after parse_early_param() and efi_memblock_x86_reserve_range().

Since we anyway do not have memblock allocations before
e820__memblock_setup(), the simplest thing would be to put the reservation
first thing in e820__memblock_setup().

-- 
Sincerely yours,
Mike.