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>,
Brijesh Singh <brijesh.singh@amd.com>,
Mike Rapoport <rppt@kernel.org>,
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 14:30:24 +0300 [thread overview]
Message-ID: <20220413113024.ycvocn6ynerl3b7m@box.shutemov.name> (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:
> >> Can we simply automate this using a kthread or smth like that, which
> >> just traverses the free page lists and accepts pages (similar, but
> >> different to free page reporting)?
> >
> > That's definitely doable.
> >
> > The downside is that this will force premature consumption of physical
> > memory resources that the guest may never use. That's a particular
> > problem on TDX systems since there is no way for a VMM to reclaim guest
> > memory short of killing the guest.
>
> IIRC, the hypervisor will usually effectively populate all guest RAM
> either way right now.
No, it is not usual. By default QEMU/KVM uses anonymous mapping and
fault-in memory on demand.
Yes, there's an option to pre-populate guest memory, but it is not the
default.
> So yes, for hypervisors that might optimize for
> that, that statement would be true. But I lost track how helpful it
> would be in the near future e.g., with the fd-based private guest memory
> -- maybe they already optimize for delayed acceptance of memory, turning
> it into delayed population.
>
> >
> > In other words, I can see a good argument either way:
> > 1. The kernel should accept everything to avoid the perf nastiness
> > 2. The kernel should accept only what it needs in order to reduce memory
> > use
> >
> > I'm kinda partial to #1 though, if I had to pick only one.
> >
> > 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.
Right. It deferred page init has to be done before init.
> 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.
Conceptionally, 3 is not different from what happens now. The first time
normal VM touches the page (like on handling __GFP_ZERO) the page gets
allocated on host. It can take very long time if it kicks in direct
reclaim on the host.
The only difference is that it is *usually* slower.
I guest we can make a case for making 1 an option to match pre-populated
use case for normal VMs.
Frankly, I think option 2 is the worst one. You still CPU cycles from the
workload after boot to do the job that may or may not be needed. It is an
half-measure that helps nobody.
--
Kirill A. Shutemov
next prev parent reply other threads:[~2022-04-13 11:28 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-05 23:43 [PATCHv4 0/8] mm, x86/cc: Implement support for unaccepted memory Kirill A. Shutemov
2022-04-05 23:43 ` [PATCHv4 1/8] mm: Add " Kirill A. Shutemov
2022-04-08 18:55 ` Dave Hansen
2022-04-09 15:54 ` Kirill A. Shutemov
2022-04-11 6:38 ` Dave Hansen
2022-04-11 10:07 ` Mike Rapoport
2022-04-13 11:40 ` Kirill A. Shutemov
2022-04-13 14:48 ` Mike Rapoport
2022-04-13 15:15 ` Kirill A. Shutemov
2022-04-13 20:06 ` Mike Rapoport
2022-04-11 8:47 ` David Hildenbrand
2022-04-08 19:04 ` David Hildenbrand
2022-04-08 19:11 ` Dave Hansen
2022-04-09 17:52 ` Kirill A. Shutemov
2022-04-11 6:41 ` Dave Hansen
2022-04-11 15:55 ` Borislav Petkov
2022-04-11 16:27 ` Dave Hansen
2022-04-11 18:55 ` Tom Lendacky
2022-04-12 8:15 ` David Hildenbrand
2022-04-12 16:08 ` Dave Hansen
2022-04-13 10:36 ` David Hildenbrand
2022-04-13 11:30 ` Kirill A. Shutemov [this message]
2022-04-13 11:32 ` David Hildenbrand
2022-04-13 15:36 ` Dave Hansen
2022-04-13 16:07 ` David Hildenbrand
2022-04-13 16:13 ` Dave Hansen
2022-04-13 16:24 ` Kirill A. Shutemov
2022-04-13 14:39 ` Mike Rapoport
2022-04-05 23:43 ` [PATCHv4 2/8] efi/x86: Get full memory map in allocate_e820() Kirill A. Shutemov
[not found] ` <Ylae+bejPzRMPrDw@zn.tnic>
2022-04-13 11:45 ` Kirill A. Shutemov
2022-04-05 23:43 ` [PATCHv4 3/8] efi/x86: Implement support for unaccepted memory Kirill A. Shutemov
2022-04-08 17:26 ` Dave Hansen
2022-04-09 19:41 ` Kirill A. Shutemov
2022-04-14 15:55 ` Borislav Petkov
2022-04-15 22:24 ` Borislav Petkov
2022-04-18 15:55 ` Kirill A. Shutemov
2022-04-18 16:38 ` Borislav Petkov
2022-04-18 20:24 ` Kirill A. Shutemov
2022-04-18 21:01 ` Borislav Petkov
2022-04-18 23:50 ` Kirill A. Shutemov
2022-04-19 7:39 ` Borislav Petkov
2022-04-19 15:30 ` Kirill A. Shutemov
2022-04-19 16:38 ` Dave Hansen
2022-04-19 19:23 ` Borislav Petkov
2022-04-21 12:26 ` Borislav Petkov
2022-04-22 0:21 ` Kirill A. Shutemov
2022-04-22 9:30 ` Borislav Petkov
2022-04-22 13:26 ` Kirill A. Shutemov
2022-04-05 23:43 ` [PATCHv4 4/8] x86/boot/compressed: Handle " Kirill A. Shutemov
2022-04-08 17:57 ` Dave Hansen
2022-04-09 20:20 ` Kirill A. Shutemov
2022-04-11 6:49 ` Dave Hansen
2022-04-05 23:43 ` [PATCHv4 5/8] x86/mm: Reserve unaccepted memory bitmap Kirill A. Shutemov
2022-04-08 18:08 ` Dave Hansen
2022-04-09 20:43 ` Kirill A. Shutemov
2022-04-05 23:43 ` [PATCHv4 6/8] x86/mm: Provide helpers for unaccepted memory Kirill A. Shutemov
2022-04-08 18:15 ` Dave Hansen
2022-04-08 19:21 ` Dave Hansen
2022-04-13 16:08 ` Kirill A. Shutemov
2022-04-05 23:43 ` [PATCHv4 7/8] x86/tdx: Unaccepted memory support Kirill A. Shutemov
2022-04-08 18:28 ` Dave Hansen
2022-04-05 23:43 ` [PATCHv4 8/8] mm/vmstat: Add counter for memory accepting Kirill A. Shutemov
2022-04-12 8:18 ` David Hildenbrand
2022-04-08 17:02 ` [PATCHv4 0/8] mm, x86/cc: Implement support for unaccepted memory Dave Hansen
2022-04-09 23:44 ` Kirill A. Shutemov
2022-04-21 12:29 ` Borislav Petkov
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