Re: [PATCHv2 1/7] mm: Add support for unaccepted memory

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: David Hildenbrand <david@redhat.com>
Cc: Dave Hansen <dave.hansen@intel.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Joerg Roedel <jroedel@suse.de>, Ard Biesheuvel <ardb@kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Varad Gautam <varad.gautam@suse.com>,
	Dario Faggioli <dfaggioli@suse.com>,
	x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 1/7] mm: Add support for unaccepted memory
Date: Wed, 12 Jan 2022 22:15:10 +0300	[thread overview]
Message-ID: <20220112191510.6uqdflbreuet7bnx@box.shutemov.name> (raw)
In-Reply-To: <d84e242f-f7f6-4fb0-8f22-bcc790a296a0@redhat.com>

On Wed, Jan 12, 2022 at 12:31:10PM +0100, David Hildenbrand wrote:
> 
> > 
> > Looking at stuff like this, I can't help but think that a:
> > 
> > 	#define PageOffline PageUnaccepted
> > 
> > and some other renaming would be a fine idea.  I get that the Offline 
> > bit can be reused, but I'm not sure that the "Offline" *naming* should 
> > be reused.  What you're doing here is logically distinct from existing 
> > offlining.
> 
> Yes, or using a new pagetype bit to make the distinction clearer.
> Especially the function names like maybe_set_page_offline() et. Al are
> confusing IMHO. They are all about accepting unaccepted memory ... and
> should express that.

"Unaccepted" is UEFI treminology and I'm not sure we want to expose
core-mm to it. Power/S390/ARM may have a different name for the same
concept. Offline/online is neutral terminology, familiar to MM developers.

What if I change accept->online in function names and document the meaning
properly?

> I assume PageOffline() will be set only on the first sub-page of a
> high-order PageBuddy() page, correct?
> 
> Then we'll have to monitor all PageOffline() users such that they can
> actually deal with PageBuddy() pages spanning *multiple* base pages for
> a PageBuddy() page. For now it's clear that if a page is PageOffline(),
> it cannot be PageBuddy() and cannot span more than one base page.

> E.g., fs/proc/kcore.c:read_kcore() assumes that PageOffline() is set on
> individual base pages.

Right, pages that offline from hotplug POV are never on page allocator's
free lists, so it cannot ever step on them.

-- 
 Kirill A. Shutemov