linux-kernel - Re: [PATCH] mm: Fix the TLB range flushed when __tlb_remove

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Date:	Wed, 29 May 2013 15:51:58 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Max Filippov <jcmvbkbc@...il.com>
Subject: Re: [PATCH] mm: Fix the TLB range flushed when __tlb_remove_page()
 runs out of slots

On Wed, May 29, 2013 at 03:36:02PM +0100, Vineet Gupta wrote:
> On 05/29/2013 07:59 PM, Catalin Marinas wrote:
> > On Wed, May 29, 2013 at 03:08:37PM +0100, Vineet Gupta wrote:
> >> On 05/29/2013 07:33 PM, Catalin Marinas wrote:
> >>> On Wed, May 29, 2013 at 01:56:13PM +0100, Vineet Gupta wrote:
> >>>> zap_pte_range loops from @addr to @end. In the middle, if it runs out of
> >>>> batching slots, TLB entries needs to be flushed for @start to @interim,
> >>>> NOT @interim to @end.
> >>>>
> >>>> Since ARC port doesn't use page free batching I can't test it myself but
> >>>> this seems like the right thing to do.
> >>>> Observed this when working on a fix for the issue at thread:
> >>>> 	http://www.spinics.net/lists/linux-arch/msg21736.html
> >>>>
> >>>> Signed-off-by: Vineet Gupta <vgupta@...opsys.com>
> >>>> Cc: Andrew Morton <akpm@...ux-foundation.org>
> >>>> Cc: Mel Gorman <mgorman@...e.de>
> >>>> Cc: Hugh Dickins <hughd@...gle.com>
> >>>> Cc: Rik van Riel <riel@...hat.com>
> >>>> Cc: David Rientjes <rientjes@...gle.com>
> >>>> Cc: Peter Zijlstra <peterz@...radead.org>
> >>>> Cc: linux-mm@...ck.org
> >>>> Cc: linux-arch@...r.kernel.org <linux-arch@...r.kernel.org>
> >>>> Cc: Catalin Marinas <catalin.marinas@....com>
> >>>> Cc: Max Filippov <jcmvbkbc@...il.com>
> >>>> ---
> >>>>  mm/memory.c |    9 ++++++---
> >>>>  1 file changed, 6 insertions(+), 3 deletions(-)
> >>>>
> >>>> diff --git a/mm/memory.c b/mm/memory.c
> >>>> index 6dc1882..d9d5fd9 100644
> >>>> --- a/mm/memory.c
> >>>> +++ b/mm/memory.c
> >>>> @@ -1110,6 +1110,7 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
> >>>>  	spinlock_t *ptl;
> >>>>  	pte_t *start_pte;
> >>>>  	pte_t *pte;
> >>>> +	unsigned long range_start = addr;
> >>>>  
> >>>>  again:
> >>>>  	init_rss_vec(rss);
> >>>> @@ -1215,12 +1216,14 @@ again:
> >>>>  		force_flush = 0;
> >>>>  
> >>>>  #ifdef HAVE_GENERIC_MMU_GATHER
> >>>> -		tlb->start = addr;
> >>>> -		tlb->end = end;
> >>>> +		tlb->start = range_start;
> >>>> +		tlb->end = addr;
> >>>>  #endif
> >>>>  		tlb_flush_mmu(tlb);
> >>>> -		if (addr != end)
> >>>> +		if (addr != end) {
> >>>> +			range_start = addr;
> >>>>  			goto again;
> >>>> +		}
> >>>>  	}
> >>> Isn't this code only run if force_flush != 0? force_flush is set to
> >>> !__tlb_remove_page() and this function always returns 1 on (generic TLB)
> >>> UP since tlb_fast_mode() is 1. There is no batching on UP with the
> >>> generic TLB code.
> >> Correct ! That's why the changelog says I couldn't test it on ARC port itself :-)
> >>
> >> However based on the other discussion (Max's TLB/PTE inconsistency), as I started
> >> writing code to reuse this block to flush the TLB even for non forced case, I
> >> realized that what this is doing is incorrect and won't work for the general flushing.
> > An alternative would be to make sure the above block is always called
> > when tlb_fast_mode():
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 6dc1882..f8b1f30 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -1211,7 +1211,7 @@ again:
> >  	 * the PTE lock to avoid doing the potential expensive TLB invalidate
> >  	 * and page-free while holding it.
> >  	 */
> > -	if (force_flush) {
> > +	if (force_flush || tlb_fast_mode(tlb)) {
> >  		force_flush = 0;
> 
> I agree with tlb_fast_mode() addition (to solve Max's issue). The problem however
> is that when we hit this at the end of loop - @addr is already pointing to @end so
> range flush gets start = end - not what we really intended.

OK. So for this part your patch looks fine.

Acked-by: Catalin Marinas <catalin.marinas@....com>
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