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Tue, 26 Nov 2019 20:00:00 -0800 (PST) Received: from [100.112.92.218] ([104.133.9.106]) by smtp.gmail.com with ESMTPSA id m5sm4479407pjl.30.2019.11.26.19.59.58 (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 26 Nov 2019 19:59:59 -0800 (PST) Date: Tue, 26 Nov 2019 19:59:49 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Bharata B Rao cc: linuxppc-dev@lists.ozlabs.org, kvm-ppc@vger.kernel.org, linux-mm@kvack.org, paulus@au1.ibm.com, aneesh.kumar@linux.vnet.ibm.com, jglisse@redhat.com, cclaudio@linux.ibm.com, linuxram@us.ibm.com, sukadev@linux.vnet.ibm.com, hch@lst.de, Paul Mackerras , Andrea Arcangeli , Hugh Dickins Subject: Re: [PATCH v11 1/7] mm: ksm: Export ksm_madvise() In-Reply-To: <20191125030631.7716-2-bharata@linux.ibm.com> Message-ID: References: <20191125030631.7716-1-bharata@linux.ibm.com> <20191125030631.7716-2-bharata@linux.ibm.com> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, 25 Nov 2019, Bharata B Rao wrote: > On PEF-enabled POWER platforms that support running of secure guests, > secure pages of the guest are represented by device private pages > in the host. Such pages needn't participate in KSM merging. This is > achieved by using ksm_madvise() call which need to be exported > since KVM PPC can be a kernel module. > > Signed-off-by: Bharata B Rao > Acked-by: Paul Mackerras > Cc: Andrea Arcangeli > Cc: Hugh Dickins I can say Acked-by: Hugh Dickins to this one. But not to your 2/7 which actually makes use of it: because sadly it needs down_write(&kvm->mm->mmap_sem) for the case when it switches off VM_MERGEABLE in vma->vm_flags. That's frustrating, since I think it's the only operation for which down_read() is not good enough. I have no idea how contended that mmap_sem is likely to be, nor how many to-be-secured pages that vma is likely to contain: you might find it okay simply to go with it down_write throughout, or you might want to start out with it down_read, and only restart with down_write (then perhaps downgrade_write later) when you see VM_MERGEABLE is set. The crash you got (thanks for the link): that will be because your migrate_vma_pages() had already been applied to a page that was already being shared via KSM. But if these secure pages are expected to be few and far between, maybe you'd prefer to keep VM_MERGEABLE, and add per-page checks of some kind into mm/ksm.c, to skip over these surprising hybrids. Hugh > --- > mm/ksm.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/mm/ksm.c b/mm/ksm.c > index dbee2eb4dd05..e45b02ad3f0b 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2478,6 +2478,7 @@ int ksm_madvise(struct vm_area_struct *vma, unsigned long start, > > return 0; > } > +EXPORT_SYMBOL_GPL(ksm_madvise); > > int __ksm_enter(struct mm_struct *mm) > { > -- > 2.21.0