linux-kernel - Re: [ 08/82] mm: pmd_read_atomic: fix 32bit PAE pmd walk vs pmd

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Message-ID: <20120607210249.GI21339@redhat.com>
Date:	Thu, 7 Jun 2012 23:02:49 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Josh Boyer <jwboyer@...il.com>,
	Greg KH <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Ulrich Obergfell <uobergfe@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Petr Matousek <pmatouse@...hat.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [ 08/82] mm: pmd_read_atomic: fix 32bit PAE pmd walk vs
 pmd_populate SMP race condition

On Thu, Jun 07, 2012 at 09:04:14PM +0200, Andrea Arcangeli wrote:
>  #ifndef CONFIG_TRANSPARENT_HUGEPAGE
> +#define pmd_read_atomic pmd_read_atomic
>  static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
>  {
>  	pmdval_t ret;
> @@ -74,11 +81,6 @@ static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
>  
>  	return (pmd_t) { ret };
>  }
> -#else /* CONFIG_TRANSPARENT_HUGEPAGE */
> -static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
> -{
> -	return (pmd_t) { atomic64_read((atomic64_t *)pmdp) };
> -}
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
>  static inline void native_set_pte_atomic(pte_t *ptep, pte_t pte)

This actually wasn't safe because if the high part wasn't none and the
low part was none, the pmd_none check would fail. I sent an updated
version that will fix this hopefully for good and it turns out the
same pmd_read_atomic will work for THP=n/y without need of cmpxchg8b
even if it's slightly less obviously safe than if we used
atomic64_read for THP=y. Still safe that is...
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