linux-kernel - Re: [PATCH] x86/mm/hotplug: fix BUG_ON() after hotremove by not freeing pud v2

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Date:   Sat, 24 Jun 2017 08:45:59 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     akpm@...ux-foundation.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>
Subject: Re: [PATCH] x86/mm/hotplug: fix BUG_ON() after hotremove by not
 freeing pud v2


* Jerome Glisse <jglisse@...hat.com> wrote:

> On Wed, Jun 07, 2017 at 09:17:06PM +0300, Kirill A. Shutemov wrote:
> > On Wed, Jun 07, 2017 at 01:38:00PM -0400, Jerome Glisse wrote:
> > > > On Wed, Jun 07, 2017 at 08:03:25PM +0300, Kirill A. Shutemov wrote:
> > > > > On Wed, Jun 07, 2017 at 10:46:20AM -0400, jglisse@...hat.com wrote:
> > > > > > From: Jérôme Glisse <jglisse@...hat.com>
> > > > > > 
> > > > > > With commit af2cf278ef4f we no longer free pud so that we do not
> > > > > > have synchronize all pgd on hotremove/vfree. But the new 5 level
> > > > > > page table patchset reverted that for 4 level page table.
> > > > > > 
> > > > > > This patch restore af2cf278ef4f and disable free_pud() if we are
> > > > > > in the 4 level page table case thus avoiding BUG_ON() after hot-
> > > > > > remove.
> > > > > > 
> > > > > > af2cf278ef4f x86/mm/hotplug: Don't remove PGD entries in
> > > > > > remove_pagetable()
> > > > > > 
> > > > > > Changed since v1:
> > > > > >   - make free_pud() conditional on the number of page table
> > > > > >     level
> > > > > >   - improved commit message
> > > > > > 
> > > > > > Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> > > > > > Cc: Andy Lutomirski <luto@...nel.org>
> > > > > > Cc: Ingo Molnar <mingo@...nel.org>
> > > > > > Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > > > > Cc: Logan Gunthorpe <logang@...tatee.com>
> > > > > > > thus we now trigger a BUG_ON() l128 in sync_global_pgds()
> > > > > > >
> > > > > > > This patch remove free_pud() like in af2cf278ef4f
> > > > > > ---
> > > > > >  arch/x86/mm/init_64.c | 11 +++++++++++
> > > > > >  1 file changed, 11 insertions(+)
> > > > > > 
> > > > > > diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> > > > > > index 95651dc..61028bc 100644
> > > > > > --- a/arch/x86/mm/init_64.c
> > > > > > +++ b/arch/x86/mm/init_64.c
> > > > > > @@ -771,6 +771,16 @@ static void __meminit free_pmd_table(pmd_t
> > > > > > *pmd_start, pud_t *pud)
> > > > > >  	spin_unlock(&init_mm.page_table_lock);
> > > > > >  }
> > > > > >  
> > > > > > +/*
> > > > > > + * For 4 levels page table we do not want to free puds but for 5 levels
> > > > > > + * we should free them. This code also need to change to adapt for boot
> > > > > > + * time switching between 4 and 5 level.
> > > > > > + */
> > > > > > +#if CONFIG_PGTABLE_LEVELS == 4
> > > > > > +static inline void free_pud_table(pud_t *pud_start, p4d_t *p4d)
> > > > > > +{
> > > > > > +}
> > > > > 
> > > > > Just "if (CONFIG_PGTABLE_LEVELS > 4)" before calling free_pud_table(), but
> > > > > okay -- I'll rework it anyway for boot-time switching.
> > > > 
> > > > Err. "if (CONFIG_PGTABLE_LEVELS == 4)" obviously.
> > > 
> > > You want me to respawn a v3 or is that good enough until you finish
> > > boot time 5 level page table ?
> > 
> > It doesn't matter for me. Upto Ingo.
> 
> Andrew any news on this ? This fix a regression in 4.12 so it would be nice to
> have this fix or similar in. I can repost a v3 without inline ie directly ifdefing
> the callsite.
> 
> Note that Kyrill will rework that but i think this is 4.13 material.

Please don't #ifdef the call site or tweak the inlines - isn't what Kirill 
suggested:

	if (CONFIG_PGTABLE_LEVELS == 4)

at the call site enough to fix the bug?

BTW., how can this be a regression, if in v4.12 CONFIG_PGTABLE_LEVELS is always 4?

For CONFIG_PGTABLE_LEVELS == 5 it won't work - but we don't have 
CONFIG_PGTABLE_LEVELS == 5 upstream yet.

Thanks,

	Ingo