From: Marc Zyngier <maz@kernel.org>
To: Quentin Perret <qperret@google.com>
Cc: james.morse@arm.com, alexandru.elisei@arm.com,
suzuki.poulose@arm.com, catalin.marinas@arm.com, will@kernel.org,
linux-arm-kernel@lists.infradead.org,
kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
ardb@kernel.org, qwandor@google.com, tabba@google.com,
dbrazdil@google.com, kernel-team@android.com,
Yanan Wang <wangyanan55@huawei.com>
Subject: Re: [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings
Date: Mon, 19 Jul 2021 13:14:48 +0100 [thread overview]
Message-ID: <87lf62jy9z.wl-maz@kernel.org> (raw)
In-Reply-To: <20210719104735.3681732-4-qperret@google.com>
On Mon, 19 Jul 2021 11:47:24 +0100,
Quentin Perret <qperret@google.com> wrote:
>
> The stage-2 map walkers currently return -EAGAIN when re-creating
> identical mappings or only changing access permissions. This allows to
> optimize mapping pages for concurrent (v)CPUs faulting on the same
> page.
>
> While this works as expected when touching one page-table leaf at a
> time, this can lead to difficult situations when mapping larger ranges.
> Indeed, a large map operation can fail in the middle if an existing
> mapping is found in the range, even if it has compatible attributes,
> hence leaving only half of the range mapped.
I'm curious of when this can happen. We normally map a single leaf at
a time, and we don't have a way to map multiple leaves at once: we
either use the VMA base size or try to upgrade it to a THP, but the
result is always a single leaf entry. What changed?
> To avoid having to deal with such failures in the caller, don't
> interrupt the map operation when hitting existing PTEs, but make sure to
> still return -EAGAIN so that user_mem_abort() can mark the page dirty
> when needed.
I don't follow you here: if you return -EAGAIN for a writable mapping,
we don't account for the page to be dirty on the assumption that
nothing has been mapped. But if there is a way to map more than a
single entry and to get -EAGAIN at the same time, then we're bound to
lose data on page eviction.
Can you shed some light on this?
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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next prev parent reply other threads:[~2021-07-19 12:16 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-19 10:47 [PATCH 00/14] Track shared pages at EL2 in protected mode Quentin Perret
2021-07-19 10:47 ` [PATCH 01/14] KVM: arm64: Provide the host_stage2_try() helper macro Quentin Perret
2021-07-20 13:57 ` Fuad Tabba
2021-07-19 10:47 ` [PATCH 02/14] KVM: arm64: Optimize kvm_pgtable_stage2_find_range() Quentin Perret
2021-07-19 10:47 ` [PATCH 03/14] KVM: arm64: Continue stage-2 map when re-creating mappings Quentin Perret
2021-07-19 12:14 ` Marc Zyngier [this message]
2021-07-19 13:32 ` Quentin Perret
2021-07-20 8:26 ` Marc Zyngier
2021-07-20 11:56 ` Quentin Perret
2021-07-19 10:47 ` [PATCH 04/14] KVM: arm64: Rename KVM_PTE_LEAF_ATTR_S2_IGNORED Quentin Perret
2021-07-19 10:47 ` [PATCH 05/14] KVM: arm64: Don't overwrite ignored bits with owner id Quentin Perret
2021-07-19 12:55 ` Marc Zyngier
2021-07-19 13:39 ` Quentin Perret
2021-07-20 8:46 ` Marc Zyngier
2021-07-19 10:47 ` [PATCH 06/14] KVM: arm64: Tolerate re-creating hyp mappings to set ignored bits Quentin Perret
2021-07-20 10:17 ` Fuad Tabba
2021-07-20 10:30 ` Quentin Perret
2021-07-20 10:59 ` Fuad Tabba
2021-07-20 11:14 ` Quentin Perret
2021-07-19 10:47 ` [PATCH 07/14] KVM: arm64: Enable forcing page-level stage-2 mappings Quentin Perret
2021-07-19 14:24 ` Marc Zyngier
2021-07-19 15:36 ` Quentin Perret
2021-07-19 10:47 ` [PATCH 08/14] KVM: arm64: Add support for tagging shared pages in page-table Quentin Perret
2021-07-19 14:43 ` Marc Zyngier
2021-07-19 15:49 ` Quentin Perret
2021-07-20 10:13 ` Marc Zyngier
2021-07-20 11:48 ` Quentin Perret
2021-07-20 13:48 ` Fuad Tabba
2021-07-20 14:06 ` Quentin Perret
2021-07-21 7:34 ` Fuad Tabba
2021-07-19 10:47 ` [PATCH 09/14] KVM: arm64: Mark host bss and rodata section as shared Quentin Perret
2021-07-19 15:01 ` Marc Zyngier
2021-07-19 15:56 ` Quentin Perret
2021-07-19 10:47 ` [PATCH 10/14] KVM: arm64: Enable retrieving protections attributes of PTEs Quentin Perret
2021-07-19 10:47 ` [PATCH 11/14] KVM: arm64: Expose kvm_pte_valid() helper Quentin Perret
2021-07-21 8:20 ` Fuad Tabba
2021-07-19 10:47 ` [PATCH 12/14] KVM: arm64: Refactor pkvm_pgtable locking Quentin Perret
2021-07-21 8:37 ` Fuad Tabba
2021-07-19 10:47 ` [PATCH 13/14] KVM: arm64: Restrict hyp stage-1 manipulation in protected mode Quentin Perret
2021-07-21 10:45 ` Fuad Tabba
2021-07-21 13:35 ` Quentin Perret
2021-07-19 10:47 ` [PATCH 14/14] KVM: arm64: Prevent late calls to __pkvm_create_private_mapping() Quentin Perret
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