linux-kernel - Re: [PATCH 1/4] mm: Provide kernel parameter to allow disabling page init poisoning

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Message-ID: <CAPcyv4j301Ma8D65oMzFo9-jVdwKGHzOVHb=7u9XaxACR5RAhg@mail.gmail.com>
Date:   Tue, 11 Sep 2018 09:50:57 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        pavel.tatashin@...rosoft.com, Michal Hocko <mhocko@...e.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/4] mm: Provide kernel parameter to allow disabling page
 init poisoning

On Mon, Sep 10, 2018 at 4:43 PM, Alexander Duyck
<alexander.duyck@...il.com> wrote:
> From: Alexander Duyck <alexander.h.duyck@...el.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.