Re: [PATCH] mm: Fix the TLB range flushed when __tlb_remove_page()runs out of slots

[Catalin Marinas]

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@xxxxxxxxxxxx>
> >> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> Cc: Mel Gorman <mgorman@xxxxxxx>
> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> >> Cc: Rik van Riel <riel@xxxxxxxxxx>
> >> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> Cc: linux-mm@xxxxxxxxx
> >> Cc: linux-arch@xxxxxxxxxxxxxxx <linux-arch@xxxxxxxxxxxxxxx>
> >> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> >> Cc: Max Filippov <jcmvbkbc@xxxxxxxxx>
> >> ---
> >>  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;
 
 #ifdef HAVE_GENERIC_MMU_GATHER

> Ignoring all other threads, do we agree that the exiting code - if used in any
> situations is incorrect semantically ?

It is incorrect unless there are requirements for
arch_leave_lazy_mmu_mode() to handle the TLB invalidation (it doesn't
look like it's widely implemented though).

-- 
Catalin
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