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Date:   Mon, 27 Nov 2023 12:13:59 +0100
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Subject: Re: [PATCH v13 17/35] KVM: Add transparent hugepage support for
 dedicated guest memory
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To:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     Xiaoyao Li <xiaoyao.li@intel.com>, Marc Zyngier <maz@kernel.org>,
        Oliver Upton <oliver.upton@linux.dev>,
        Huacai Chen <chenhuacai@kernel.org>,
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        Anish Moorthy <amoorthy@google.com>,
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From:   Vlastimil Babka <vbabka@suse.cz>
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On 11/2/23 16:46, Paolo Bonzini wrote:
> On Thu, Nov 2, 2023 at 4:38 PM Sean Christopherson <seanjc@google.com> wrote:
>> Actually, looking that this again, there's not actually a hard dependency on THP.
>> A THP-enabled kernel _probably_  gives a higher probability of using hugepages,
>> but mostly because THP selects COMPACTION, and I suppose because using THP for
>> other allocations reduces overall fragmentation.
> 
> Yes, that's why I didn't even bother enabling it unless THP is
> enabled, but it makes even more sense to just try.
> 
>> So rather than honor KVM_GUEST_MEMFD_ALLOW_HUGEPAGE iff THP is enabled, I think
>> we should do the below (I verified KVM can create hugepages with THP=n).  We'll
>> need another capability, but (a) we probably should have that anyways and (b) it
>> provides a cleaner path to adding PUD-sized hugepage support in the future.
> 
> I wonder if we need KVM_CAP_GUEST_MEMFD_HUGEPAGE_PMD_SIZE though. This
> should be a generic kernel API and in fact the sizes are available in
> a not-so-friendly format in /sys/kernel/mm/hugepages.
> 
> We should just add /sys/kernel/mm/hugepages/sizes that contains
> "2097152 1073741824" on x86 (only the former if 1G pages are not
> supported).
> 
> Plus: is this the best API if we need something else for 1G pages?
> 
> Let's drop *this* patch and proceed incrementally. (Again, this is
> what I want to do with this final review: identify places that are
> stil sticky, and don't let them block the rest).
> 
> Coincidentially we have an open spot next week at plumbers. Let's
> extend Fuad's section to cover more guestmem work.

Hi,

was there any outcome wrt this one? Based on my experience with THP's it
would be best if userspace didn't have to opt-in, nor care about the
supported size. If the given size is unaligned, provide a mix of large pages
up to an aligned size, and for the rest fallback to base pages, which should
be better than -EINVAL on creation (is it possible with the current
implementation? I'd hope so so?). A way to opt-out from huge pages could be
useful although there's always the risk of some initial troubles resulting
in various online sources cargo-cult recommending to opt-out forever.

Vlastimil