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Date:   Fri, 4 Jun 2021 15:49:30 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Sean Christopherson <seanjc@google.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Andrea Arcangeli <aarcange@redhat.com>
Cc:     linux-mm@kvack.org, Paolo Bonzini <pbonzini@redhat.com>,
        kvm@vger.kernel.org
Subject: PageTransCompoundMap confusion
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I'm a bit confused about what PageTransCompoundMap() is supposed to do.
What it actually does is check that the specific page (which may or
may not be a head page) is not mapped by a PTE.  I don't understand why
you'd care how some (other?) process does or does not have it mapped.
What I _think_ you want to know is "Can I map this page with a PMD entry
in the guest".  And the answer to that is simply:

bool kvm_is_transparent_hugepage(kvm_pfn_t pfn)
{
	struct page *head = compound_head(pfn_to_page(pfn));
	return compound_order(head) >= HPAGE_PMD_ORDER;
}

but maybe there's some reason you don't want to map hugetlbfs or other
sufficiently large compound pages with PMDs?

Looking at the one caller of kvm_is_transparent_hugepage(), I'd be
tempted to inline the above into transparent_hugepage_adjust()
and call get_page() directly instead of indirecting through
kvm_get_pfn().