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Date:   Tue, 31 Oct 2023 07:16:15 -0700
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 <7c0844d8-6f97-4904-a140-abeabeb552c1@intel.com>
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Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory
From:   Sean Christopherson <seanjc@google.com>
To:     Xiaoyao Li <xiaoyao.li@intel.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Anup Patel <anup@brainfault.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Xu Yilun <yilun.xu@intel.com>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Fuad Tabba <tabba@google.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Anish Moorthy <amoorthy@google.com>,
        David Matlack <dmatlack@google.com>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        Isaku Yamahata <isaku.yamahata@intel.com>,
        "=?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?=" <mic@digikod.net>,
        Vlastimil Babka <vbabka@suse.cz>,
        Vishal Annapurve <vannapurve@google.com>,
        Ackerley Tng <ackerleytng@google.com>,
        Maciej Szmigiero <mail@maciej.szmigiero.name>,
        David Hildenbrand <david@redhat.com>,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>,
        Wang <wei.w.wang@intel.com>,
        Liam Merwick <liam.merwick@oracle.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
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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