Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

From: Mike Rapoport <rppt@kernel.org>
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>,
	Brijesh Singh <brijesh.singh@amd.com>,
	x86@kernel.org, linux-mm@kvack.org, linux-coco@lists.linux.dev,
	linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org,
	Mike Rapoport <rppt@linux.ibm.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory
Date: Wed, 13 Apr 2022 17:39:20 +0300	[thread overview]
Message-ID: <YlbgmOwb9uXMt49l@kernel.org> (raw)
In-Reply-To: <a458c13f-9994-b227-ff61-bfdfec10bc27@redhat.com>

On Wed, Apr 13, 2022 at 12:36:11PM +0200, David Hildenbrand wrote:
> On 12.04.22 18:08, Dave Hansen wrote:
> > On 4/12/22 01:15, David Hildenbrand wrote:
> > 
> > The other option might be to tie this all to DEFERRED_STRUCT_PAGE_INIT.
> >  Have the rule that everything that gets a 'struct page' must be
> > accepted.  If you want to do delayed acceptance, you do it via
> > DEFERRED_STRUCT_PAGE_INIT.
> 
> That could also be an option, yes. At least being able to chose would be
> good. But IIRC, DEFERRED_STRUCT_PAGE_INIT will still make the system get
> stuck during boot and wait until everything was accepted.

The deferred page init runs multithreaded, so guest with SMP will be stuck
for less time.

> I see the following variants:
> 
> 1) Slow boot; after boot, all memory is already accepted.
> 2) Fast boot; after boot, all memory will slowly but steadily get
>    accepted in the background. After a while, all memory is accepted and
>    can be signaled to user space.
> 3) Fast boot; after boot, memory gets accepted on demand. This is what
>    we have in this series.
> 
> I somehow don't quite like 3), but with deferred population in the
> hypervisor, it might just make sense.

IMHO, deferred population in hypervisor will be way more complex than this
series with similar "visible" performance.

-- 
Sincerely yours,
Mike.