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From: Dan Williams <dan.j.williams@intel.com>
Date: Tue, 11 Sep 2018 09:50:57 -0700
Message-ID: <CAPcyv4j301Ma8D65oMzFo9-jVdwKGHzOVHb=7u9XaxACR5RAhg@mail.gmail.com>
Subject: Re: [PATCH 1/4] mm: Provide kernel parameter to allow disabling page
 init poisoning
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Sender: owner-linux-mm@kvack.org
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To: Alexander Duyck <alexander.duyck@gmail.com>
Cc: Linux MM <linux-mm@kvack.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-nvdimm <linux-nvdimm@lists.01.org>, pavel.tatashin@microsoft.com, Michal Hocko <mhocko@suse.com>, Dave Jiang <dave.jiang@intel.com>, Ingo Molnar <mingo@kernel.org>, Dave Hansen <dave.hansen@intel.com>, =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Logan Gunthorpe <logang@deltatee.com>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

On Mon, Sep 10, 2018 at 4:43 PM, Alexander Duyck
<alexander.duyck@gmail.com> wrote:
> From: Alexander Duyck <alexander.h.duyck@intel.com>
>
> On systems with a large amount of memory it can take a significant amount
> of time to initialize all of the page structs with the PAGE_POISON_PATTERN
> value. I have seen it take over 2 minutes to initialize a system with
> over 12GB of RAM.
>
> In order to work around the issue I had to disable CONFIG_DEBUG_VM and then
> the boot time returned to something much more reasonable as the
> arch_add_memory call completed in milliseconds versus seconds. However in
> doing that I had to disable all of the other VM debugging on the system.
>
> In order to work around a kernel that might have CONFIG_DEBUG_VM enabled on
> a system that has a large amount of memory I have added a new kernel
> parameter named "page_init_poison" that can be set to "off" in order to
> disable it.

In anticipation of potentially more DEBUG_VM options wanting runtime
control I'd propose creating a new "vm_debug=" option for this modeled
after "slub_debug=" along with a CONFIG_DEBUG_VM_ON to turn on all
options.

That way there is more differentiation for debug cases like this that
have significant performance impact when enabled.

CONFIG_DEBUG_VM leaves optional debug capabilities disabled by default
unless CONFIG_DEBUG_VM_ON is also set.