Re: [PATCH v1 09/10] mm/memory_hotplug: Don't mark pages PG_reserved when initializing the memmap

From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
To: David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Cc: linux-hyperv@vger.kernel.org, "Michal Hocko" <mhocko@suse.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	kvm@vger.kernel.org,
	"Pavel Tatashin" <pavel.tatashin@microsoft.com>,
	"KarimAllah Ahmed" <karahmed@amazon.de>,
	"Dave Hansen" <dave.hansen@linux.intel.com>,
	"Alexander Duyck" <alexander.duyck@gmail.com>,
	"Michal Hocko" <mhocko@kernel.org>,
	linux-mm@kvack.org, "Paul Mackerras" <paulus@samba.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Wanpeng Li" <wanpengli@tencent.com>,
	"Alexander Duyck" <alexander.h.duyck@linux.intel.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Kees Cook" <keescook@chromium.org>,
	devel@driverdev.osuosl.org,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Stephen Hemminger" <sthemmin@microsoft.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	"Joerg Roedel" <joro@8bytes.org>,
	x86@kernel.org, YueHaibing <yuehaibing@huawei.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Mike Rapoport" <rppt@linux.ibm.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Ingo Molnar" <mingo@redhat.com>,
	"Vlastimil Babka" <vbabka@suse.cz>,
	"Anthony Yznaga" <anthony.yznaga@oracle.com>,
	"Oscar Salvador" <osalvador@suse.de>,
	"Isaac J. Manjarres" <isaacm@codeaurora.org>,
	"Matt Sickler" <Matt.Sickler@daktronics.com>,
	"Juergen Gross" <jgross@suse.com>,
	"Anshuman Khandual" <anshuman.khandual@arm.com>,
	"Haiyang Zhang" <haiyangz@microsoft.com>,
	"Sasha Levin" <sashal@kernel.org>,
	kvm-ppc@vger.kernel.org, "Qian Cai" <cai@lca.pw>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Mike Rapoport" <rppt@linux.vnet.ibm.com>,
	"Borislav Petkov" <bp@alien8.de>,
	"Nicholas Piggin" <npiggin@gmail.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	xen-devel@lists.xenproject.org,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Vitaly Kuznetsov" <vkuznets@redhat.com>,
	"Allison Randal" <allison@lohutok.net>,
	"Jim Mattson" <jmattson@google.com>,
	"Mel Gorman" <mgorman@techsingularity.net>,
	"Cornelia Huck" <cohuck@redhat.com>,
	"Pavel Tatashin" <pasha.tatashin@soleen.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Johannes Weiner" <hannes@cmpxchg.org>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v1 09/10] mm/memory_hotplug: Don't mark pages PG_reserved when initializing the memmap
Date: Mon, 4 Nov 2019 17:44:33 -0500
Message-ID: <4b88ebd7-255d-4f02-a347-5a6c0f4f4ac4@oracle.com> (raw)
In-Reply-To: <20191024120938.11237-10-david@redhat.com>

On 10/24/19 8:09 AM, David Hildenbrand wrote:
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index 4f2e78a5e4db..af69f057913a 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -374,6 +374,13 @@ static void xen_online_page(struct page *page, unsigned int order)
>  	mutex_lock(&balloon_mutex);
>  	for (i = 0; i < size; i++) {
>  		p = pfn_to_page(start_pfn + i);
> +		/*
> +		 * TODO: The core used to mark the pages reserved. Most probably
> +		 * we can stop doing that now. However, especially
> +		 * alloc_xenballooned_pages() left PG_reserved set
> +		 * on pages that can get mapped to user space.
> +		 */
> +		__SetPageReserved(p);

I suspect this is not needed. Pages can get into balloon either from
here or from non-hotplug path (e.g. decrease_reservation()) and so when
we get a page from the balloon we would get a random page that may or
may not have Reserved bit set.

-boris

>  		balloon_append(p);
>  	}
>  	mutex_unlock(&balloon_mutex);
>