linux-kernel - Re: [PATCH v2 1/2] mm: Move page struct poisoning to CONFIG_DEBUG_VM_PAGE_INIT

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Date:   Wed, 5 Sep 2018 14:29:07 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Pavel.Tatashin@...rosoft.com
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v2 1/2] mm: Move page struct poisoning to CONFIG_DEBUG_VM_PAGE_INIT_POISON

On Wed, Sep 5, 2018 at 2:22 PM Pasha Tatashin
<Pavel.Tatashin@...rosoft.com> wrote:
>
>
>
> On 9/5/18 5:13 PM, Alexander Duyck 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.
> >
> > Instead of keeping the value in CONFIG_DEBUG_VM I am adding a new CONFIG
> > value called CONFIG_DEBUG_VM_PAGE_INIT_POISON that will control the page
> > poisoning independent of the CONFIG_DEBUG_VM option.
> >
> > Signed-off-by: Alexander Duyck <alexander.h.duyck@...el.com>
> > ---
> >  include/linux/page-flags.h |    8 ++++++++
> >  lib/Kconfig.debug          |   14 ++++++++++++++
> >  mm/memblock.c              |    5 ++---
> >  mm/sparse.c                |    4 +---
> >  4 files changed, 25 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 74bee8cecf4c..0e95ca63375a 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -13,6 +13,7 @@
> >  #include <linux/mm_types.h>
> >  #include <generated/bounds.h>
> >  #endif /* !__GENERATING_BOUNDS_H */
> > +#include <linux/string.h>
> >
> >  /*
> >   * Various page->flags bits:
> > @@ -162,6 +163,13 @@ static inline int PagePoisoned(const struct page *page)
> >       return page->flags == PAGE_POISON_PATTERN;
> >  }
> >
> > +static inline void page_init_poison(struct page *page, size_t size)
> > +{
> > +#ifdef CONFIG_DEBUG_VM_PAGE_INIT_POISON
> > +     memset(page, PAGE_POISON_PATTERN, size);
> > +#endif
> > +}
> > +
> >  /*
> >   * Page flags policies wrt compound pages
> >   *
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 613316724c6a..3b1277c52fed 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -637,6 +637,20 @@ config DEBUG_VM_PGFLAGS
> >
> >         If unsure, say N.
> >
> > +config DEBUG_VM_PAGE_INIT_POISON
> > +     bool "Enable early page metadata poisoning"
> > +     default y
> > +     depends on DEBUG_VM
> > +     help
> > +       Seed the page metadata with a poison pattern to improve the
> > +       likelihood of detecting attempts to access the page prior to
> > +       initialization by the memory subsystem.
> > +
> > +       This initialization can result in a longer boot time for systems
> > +       with a large amount of memory.
>
> What happens when DEBUG_VM_PGFLAGS = y and
> DEBUG_VM_PAGE_INIT_POISON = n ?
>
> We are testing for pattern that was not set?
>
> I think DEBUG_VM_PAGE_INIT_POISON must depend on DEBUG_VM_PGFLAGS instead.
>
> Looks good otherwise.
>
> Thank you,
> Pavel

The problem is that I then end up in the same situation I had in the
last patch where you have to have DEBUG_VM_PGFLAGS on in order to do
the seeding with poison.

I can wrap the bit of code in PagePoisoned to just always return false
if we didn't set the pattern. I figure there is value to be had for
running DEBUG_VM_PGFLAGS regardless of the poison check, or
DEBUG_VM_PAGE_INIT_POISON without the PGFLAGS check. That is why I
wanted to leave them independent.

- Alex