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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 11 Sep 2018 13:24:29 -0700
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Subject: Re: [PATCH 1/4] mm: Provide kernel parameter to allow disabling page
 init poisoning
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     linux-mm <linux-mm@kvack.org>, LKML <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>
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On Tue, Sep 11, 2018 at 1:01 PM, Alexander Duyck
<alexander.duyck@gmail.com> wrote:
> On Tue, Sep 11, 2018 at 9:50 AM Dan Williams <dan.j.williams@intel.com> wrote:
>>
>> 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.
>
> Based on earlier discussions I would assume that CONFIG_DEBUG_VM would
> imply CONFIG_DEBUG_VM_ON anyway since we don't want most of these
> disabled by default.
>
> In my mind we should be looking at a selective "vm_debug_disable="
> instead of something that would be turning on features.

Sorry, I missed those earlier discussions, so I won't push too hard if
this has been hashed before. My proposal for opt-in is the fact that
at least one known distribution kernel, Fedora, is shipping with
CONFIG_DEBUG_VM=y. They also ship with CONFIG_SLUB, but not
SLUB_DEBUG_ON. If we are going to picemeal enable some debug options
to be runtime controlled I think we should go further to start
clarifying the cheap vs the expensive checks and making the expensive
checks opt-in in the same spirit of SLUB_DEBUG.