From: Christoffer Dall <christoffer.dall@arm.com>
To: kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>, kvmarm@lists.cs.columbia.edu
Subject: Reference count on pages held in secondary MMUs
Date: Sun, 9 Jun 2019 10:18:05 +0200 [thread overview]
Message-ID: <20190609081805.GC21798@e113682-lin.lund.arm.com> (raw)
Hi,
I have been looking at how we deal with page_count(page) on pages held
in stage 2 page tables on KVM/arm64.
What we do currently is to drop the reference on the page we get from
get_user_pages() once the page is inserted into our stage 2 page table,
typically leaving page_count(page) == page_mapcount(page) == 1 which
represents the userspace stage 1 mapping of the page,
and we rely on MMU notifiers to remove the stage 2 mapping if
corresponding stage 1 mapping is being unmapped.
I believe this is analogous to what other architectures do?
In some sense, we are thus maintaining a 'hidden', or internal,
reference to the page, which is not counted anywhere.
I am wondering if it would be equally valid to take a reference on the
page, and remove that reference when unmapping via MMU notifiers, and if
so, if there would be any advantages/drawbacks in doing so?
Thanks,
Christoffer
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next reply other threads:[~2019-06-09 8:18 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-09 8:18 Christoffer Dall [this message]
2019-06-09 9:37 ` Reference count on pages held in secondary MMUs Paolo Bonzini
2019-06-09 17:40 ` Andrea Arcangeli
2019-06-11 11:51 ` Christoffer Dall
2019-06-22 19:11 ` Andrea Arcangeli
2019-06-26 12:16 ` Christoffer Dall
2019-06-11 11:21 ` Christoffer Dall
2019-06-11 11:29 ` Paolo Bonzini
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