linux-kernel - Re: [RFC][PATCH 2/3] track numbers of pagetable pages

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Message-ID: <1303138924.9615.2487.camel@nimitz>
Date:	Mon, 18 Apr 2011 08:02:04 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Matt Fleming <matt@...sole-pimps.org>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 2/3] track numbers of pagetable pages

On Sat, 2011-04-16 at 10:44 +0100, Matt Fleming wrote:
> >  static inline void pgtable_page_dtor(struct mm_struct *mm, struct page *page)
> >  {
> >  	pte_lock_deinit(page);
> > +	dec_mm_counter(mm, MM_PTEPAGES);
> >  	dec_zone_page_state(page, NR_PAGETABLE);
> >  }
> 
> I'm probably missing something really obvious but...
> 
> Is this safe in the non-USE_SPLIT_PTLOCKS case? If we're not using
> split-ptlocks then inc/dec_mm_counter() are only safe when done under
> mm->page_table_lock, right? But it looks to me like we can end up doing,
> 
>   __pte_alloc()
>       pte_alloc_one()
>           pgtable_page_ctor()
> 
> before acquiring mm->page_table_lock in __pte_alloc().

No, it's probably not safe.  We'll have to come up with something a bit
different in that case.  Either that, or just kill the non-atomic case.
Surely there's some percpu magic counter somewhere in the kernel that is
optimized for fast (unlocked?) updates and rare, slow reads.

-- Dave

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