linux-kernel - Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for dedicated guest memory

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Message-ID: <92ba7ddd-2bc8-4a8d-bd67-d6614b21914f@intel.com>
Date:   Wed, 1 Nov 2023 15:25:23 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        David Matlack <dmatlack@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        Mickaël Salaün <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory

On 10/31/2023 10:16 PM, Sean Christopherson wrote:
> On Tue, Oct 31, 2023, Xiaoyao Li wrote:
>> On 10/28/2023 2:21 AM, Sean Christopherson wrote:
>>> Extended guest_memfd to allow backing guest memory with transparent
>>> hugepages. Require userspace to opt-in via a flag even though there's no
>>> known/anticipated use case for forcing small pages as THP is optional,
>>> i.e. to avoid ending up in a situation where userspace is unaware that
>>> KVM can't provide hugepages.
>>
>> Personally, it seems not so "transparent" if requiring userspace to opt-in.
>>
>> People need to 1) check if the kernel built with TRANSPARENT_HUGEPAGE
>> support, or check is the sysfs of transparent hugepage exists; 2)get the
>> maximum support hugepage size 3) ensure the size satisfies the alignment;
>> before opt-in it.
>>
>> Even simpler, userspace can blindly try to create guest memfd with
>> transparent hugapage flag. If getting error, fallback to create without the
>> transparent hugepage flag.
>>
>> However, it doesn't look transparent to me.
> 
> The "transparent" part is referring to the underlying kernel mechanism, it's not
> saying anything about the API.  The "transparent" part of THP is that the kernel
> doesn't guarantee hugepages, i.e. whether or not hugepages are actually used is
> (mostly) transparent to userspace.
> 
> Paolo also isn't the biggest fan[*], but there are also downsides to always
> allowing hugepages, e.g. silent failure due to lack of THP or unaligned size,
> and there's precedent in the form of MADV_HUGEPAGE.
> 
> [*] https://lore.kernel.org/all/84a908ae-04c7-51c7-c9a8-119e1933a189@redhat.com

But it's different than MADV_HUGEPAGE, in a way. Per my understanding, 
the failure of MADV_HUGEPAGE is not fatal, user space can ignore it and 
continue.

However, the failure of KVM_GUEST_MEMFD_ALLOW_HUGEPAGE is fatal, which 
leads to failure of guest memfd creation.

For current implementation, I think maybe 
KVM_GUEST_MEMFD_DESIRE_HUGEPAGE fits better than 
KVM_GUEST_MEMFD_ALLOW_HUGEPAGE? or maybe *PREFER*?