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

From: Alexander Duyck <alexander.duyck@gmail.com>
To: mhocko@kernel.org
Cc: pavel.tatashin@microsoft.com, linux-nvdimm@lists.01.org,
	Dave Hansen <dave.hansen@intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	jglisse@redhat.com,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 1/4] mm: Provide kernel parameter to allow disabling page init poisoning
Date: Wed, 12 Sep 2018 07:49:06 -0700	[thread overview]
Message-ID: <CAKgT0UdvhV7U5Zniq=KskXz2QsRP8C7ctr5=ZtJwYAVpBT-RHw@mail.gmail.com> (raw)
In-Reply-To: <20180912141053.GL10951@dhcp22.suse.cz>

On Wed, Sep 12, 2018 at 7:10 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 10-09-18 16:43:41, Alexander Duyck 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.
>
> I am still not convinced that this all is worth the additional code. It
> is much better than a new config option for sure. If we really want this
> though then I suggest that the parameter handler should note the
> disabled state (when CONFIG_DEBUG_VM is on) to the kernel log. I would
> also make it explicit who might want to do that in the parameter
> description.

Anything specific in terms of the kernel log message we are looking
for? I'll probably just go with "Page struct poisoning disabled by
kernel command line option 'page_init_poison'" or something along
those lines.

> > +     page_init_poison=       [KNL] Boot-time parameter changing the
> > +                     state of poisoning of page structures during early
> > +                     boot. Used to verify page metadata is not accessed
> > +                     prior to initialization. Available with
> > +                     CONFIG_DEBUG_VM=y.
> > +                     off: turn off poisoning
> > +                     on: turn on poisoning (default)
> > +
>
> what about the following wording or something along those lines
>
> Boot-time parameter to control struct page poisoning which is a
> debugging feature to catch unitialized struct page access. This option
> is available only for CONFIG_DEBUG_VM=y and it affects boot time
> (especially on large systems). If there are no poisoning bugs reported
> on the particular system and workload it should be safe to disable it to
> speed up the boot time.

That works for me. I will update it for the next release.

Thanks.

- Alex
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