Message-ID: <98b8ea1e-3bdd-4987-8acc-58112076d451@amazon.com>
Date: Fri, 16 Jan 2026 17:51:24 +0000
From: Nikita Kalyazin <kalyazin@...zon.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "kernel@...0n.name"
	<kernel@...0n.name>, "linux-riscv@...ts.infradead.org"
	<linux-riscv@...ts.infradead.org>, "linux-s390@...r.kernel.org"
	<linux-s390@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
	"kalyazin@...zon.co.uk" <kalyazin@...zon.co.uk>, "kvmarm@...ts.linux.dev"
	<kvmarm@...ts.linux.dev>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>, "loongarch@...ts.linux.dev"
	<loongarch@...ts.linux.dev>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>
CC: "david@...nel.org" <david@...nel.org>, "svens@...ux.ibm.com"
	<svens@...ux.ibm.com>, "catalin.marinas@....com" <catalin.marinas@....com>,
	"palmer@...belt.com" <palmer@...belt.com>, "jgross@...e.com"
	<jgross@...e.com>, "surenb@...gle.com" <surenb@...gle.com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "riel@...riel.com" <riel@...riel.com>, "pfalcato@...e.de"
	<pfalcato@...e.de>, "x86@...nel.org" <x86@...nel.org>, "rppt@...nel.org"
	<rppt@...nel.org>, "thuth@...hat.com" <thuth@...hat.com>,
	"borntraeger@...ux.ibm.com" <borntraeger@...ux.ibm.com>, "maz@...nel.org"
	<maz@...nel.org>, "peterx@...hat.com" <peterx@...hat.com>, "ast@...nel.org"
	<ast@...nel.org>, "Annapurve, Vishal" <vannapurve@...gle.com>,
	"pjw@...nel.org" <pjw@...nel.org>, "alex@...ti.fr" <alex@...ti.fr>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "hca@...ux.ibm.com"
	<hca@...ux.ibm.com>, "willy@...radead.org" <willy@...radead.org>,
	"wyihan@...gle.com" <wyihan@...gle.com>, "ryan.roberts@....com"
	<ryan.roberts@....com>, "jolsa@...nel.org" <jolsa@...nel.org>,
	"yang@...amperecomputing.com" <yang@...amperecomputing.com>,
	"jmattson@...gle.com" <jmattson@...gle.com>, "aneesh.kumar@...nel.org"
	<aneesh.kumar@...nel.org>, "luto@...nel.org" <luto@...nel.org>,
	"haoluo@...gle.com" <haoluo@...gle.com>, "patrick.roy@...ux.dev"
	<patrick.roy@...ux.dev>, "akpm@...ux-foundation.org"
	<akpm@...ux-foundation.org>, "coxu@...hat.com" <coxu@...hat.com>,
	"mhocko@...e.com" <mhocko@...e.com>, "mlevitsk@...hat.com"
	<mlevitsk@...hat.com>, "jgg@...pe.ca" <jgg@...pe.ca>, "hpa@...or.com"
	<hpa@...or.com>, "song@...nel.org" <song@...nel.org>, "oupton@...nel.org"
	<oupton@...nel.org>, "maobibo@...ngson.cn" <maobibo@...ngson.cn>,
	"peterz@...radead.org" <peterz@...radead.org>, "Liam.Howlett@...cle.com"
	<Liam.Howlett@...cle.com>, "lorenzo.stoakes@...cle.com"
	<lorenzo.stoakes@...cle.com>, "jhubbard@...dia.com" <jhubbard@...dia.com>,
	"martin.lau@...ux.dev" <martin.lau@...ux.dev>, "jthoughton@...gle.com"
	<jthoughton@...gle.com>, "Jonathan.Cameron@...wei.com"
	<Jonathan.Cameron@...wei.com>, "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
	"eddyz87@...il.com" <eddyz87@...il.com>, "yonghong.song@...ux.dev"
	<yonghong.song@...ux.dev>, "chenhuacai@...nel.org" <chenhuacai@...nel.org>,
	"shuah@...nel.org" <shuah@...nel.org>, "prsampat@....com" <prsampat@....com>,
	"kevin.brodsky@....com" <kevin.brodsky@....com>,
	"shijie@...amperecomputing.com" <shijie@...amperecomputing.com>,
	"itazur@...zon.co.uk" <itazur@...zon.co.uk>, "suzuki.poulose@....com"
	<suzuki.poulose@....com>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"dev.jain@....com" <dev.jain@....com>, "yuzenghui@...wei.com"
	<yuzenghui@...wei.com>, "gor@...ux.ibm.com" <gor@...ux.ibm.com>,
	"jackabt@...zon.co.uk" <jackabt@...zon.co.uk>, "daniel@...earbox.net"
	<daniel@...earbox.net>, "agordeev@...ux.ibm.com" <agordeev@...ux.ibm.com>,
	"andrii@...nel.org" <andrii@...nel.org>, "mingo@...hat.com"
	<mingo@...hat.com>, "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
	"joey.gouly@....com" <joey.gouly@....com>, "derekmn@...zon.com"
	<derekmn@...zon.com>, "xmarcalx@...zon.co.uk" <xmarcalx@...zon.co.uk>,
	"kpsingh@...nel.org" <kpsingh@...nel.org>, "sdf@...ichev.me"
	<sdf@...ichev.me>, "jackmanb@...gle.com" <jackmanb@...gle.com>,
	"bp@...en8.de" <bp@...en8.de>, "corbet@....net" <corbet@....net>,
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "jannh@...gle.com"
	<jannh@...gle.com>, "john.fastabend@...il.com" <john.fastabend@...il.com>,
	"kas@...nel.org" <kas@...nel.org>, "will@...nel.org" <will@...nel.org>,
	"seanjc@...gle.com" <seanjc@...gle.com>
Subject: Re: [PATCH v9 07/13] KVM: guest_memfd: Add flag to remove from direct
 map



On 16/01/2026 17:36, Edgecombe, Rick P wrote:
> On Fri, 2026-01-16 at 17:28 +0000, Nikita Kalyazin wrote:
>>>
>>> I imagine this feature is really targeted towards machines running
>>> a bunch of untrusted VMs, so cloud hypervisors really. In that case
>>> the direct map will probably be carved up pretty quick. Did you
>>> consider just breaking the full direct map to 4k at the start when
>>> it's in use?
>>
>> That's an interesting point, I haven't thought about it from this
>> perspective.  We should run some tests internally to see if it'd
>> help.  This will likely change with support for huge pages coming in
>> though.
> 
> The thing is, those no_flush() helpers actually still flush if they
> need to split a page. Plus if they need to clear out lazy vmalloc
> aliases it could be another flush. There are probably a lot of
> opportunities to reduce flushing even beyond pre-split.
> 
> Just curious... as far as performance, have you tested this on a big
> multi-socket system, where that flushing will hurt more? It's something
> that has always been a fear for these directmap unmapping solutions

Yes, this is a problem that we'd like to address.  We have been 
discussing it in [1].  The effect of flushing on memory population that 
we see on x86 is 5-7x elongation.  We are thinking of making use of the 
no-direct-map memory allocator that Brendan is working on [2].

[1] 
https://lore.kernel.org/lkml/d1b58114-9b88-4535-b28c-09d9cc1ff3be@amazon.com
[2] https://lore.kernel.org/kvm/DDVS9ITBCE2Z.RSTLCU79EX8G@google.com
> 

Powered by blists - more mailing lists