linux-kernel - Re: [PATCH] mm: Check if PTE is already allocated during page fault

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Message-ID: <1303111273.3425.0.camel@raz.scalemp.com>
Date:	Mon, 18 Apr 2011 10:21:13 +0300
From:	raz ben yehuda <raziebe@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Mel Gorman <mgorman@...e.de>, akpm@...ux-foundation.org,
	riel@...hat.com, kosaki.motohiro@...fujitsu.com,
	lkml <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
	stable@...nel.org, shai@...lex86.com
Subject: Re: [PATCH] mm: Check if PTE is already allocated during page fault

patch works great. thank you Andrea.


On Fri, 2011-04-15 at 17:06 +0200, Andrea Arcangeli wrote:
> On Fri, Apr 15, 2011 at 04:39:16PM +0200, Andrea Arcangeli wrote:
> > On Fri, Apr 15, 2011 at 11:12:4A.M +0100, Mel Gorman wrote:
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 5823698..1659574 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3322,7 +3322,7 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> > >  	 * run pte_offset_map on the pmd, if an huge pmd could
> > >  	 * materialize from under us from a different thread.
> > >  	 */
> > > -	if (unlikely(__pte_alloc(mm, vma, pmd, address)))
> > > +	if (unlikely(pmd_none(*pmd)) && __pte_alloc(mm, vma, pmd, address))
> 
> I started hacking on this and I noticed it'd be better to extend the
> unlikely through the end. At first review I didn't notice the
> parenthesis closure stops after pte_none and __pte_alloc is now
> uncovered. I'd prefer this:
> 
>     if (unlikely(pmd_none(*pmd) && __pte_alloc(mm, vma, pmd, address)))
> 
> I mean the real unlikely thing is that we return VM_FAULT_OOM, if we
> end up calling __pte_alloc or not, depends on the app. Generally it
> sounds more frequent that the pte is not none, so it's not wrong, but
> it's even less likely that __pte_alloc fails so that can be taken into
> account too, and __pte_alloc runs still quite frequently. So either
> above or:
> 
>     if (unlikely(pmd_none(*pmd)) && unlikely(__pte_alloc(mm, vma, pmd, address)))
> 
> I generally prefer unlikely only when it's 100% sure thing it's less
> likely (like the VM_FAULT_OOM), so the first version I guess it's
> enough (I'm afraid unlikely for pte_none too, may make gcc generate a
> far away jump possibly going out of l1 icache for a case that is only
> 512 times less likely at best). My point is that it's certainly hugely
> more unlikely that __pte_alloc fails than the pte is none.
> 
> This is a real nitpick though ;).


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