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

Vineet Gupta
2013-05-29T14:09:16Z

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@synopsys.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Mel Gorman <mgorman@suse.de>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Rik van Riel <riel@redhat.com>
>> Cc: David Rientjes <rientjes@google.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: linux-mm@kvack.org
>> Cc: linux-arch@vger.kernel.org <linux-arch@vger.kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Max Filippov <jcmvbkbc@gmail.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.

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

-Vineet