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

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Date:   Wed, 12 Jan 2022 22:43:02 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "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 Tue, Jan 11, 2022 at 11:10:40AM -0800, Dave Hansen wrote:
> On 1/11/22 03:33, Kirill A. Shutemov wrote:
> > Unaccepted memory bitmap is allocated during decompression stage and
> > handed over to main kernel image via boot_params. The bitmap is used to
> > track if memory has been accepted.
> > 
> > Reserve unaccepted memory bitmap has to prevent reallocating memory for
> > other means.
> 
> I'm having a hard time parsing that changelog, especially the second
> paragraph.  Could you give it another shot?

What about this:

	Unaccepted memory bitmap is allocated during decompression stage and
	handed over to main kernel image via boot_params.

	Kernel tracks what memory has been accepted in the bitmap.

	Reserve memory where the bitmap is placed to prevent memblock from
	re-allocating the memory for other needs.

?

> > +	/* 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.

-- 
 Kirill A. Shutemov