Re: [PATCH v2 4/5] mm, highmem: makes flush_all_zero_pkmaps() returnindex of first flushed entry

[JoonSoo Kim]

Hello, Minchan.

2012/11/1 Minchan Kim <minchan@xxxxxxxxxx>:
> On Thu, Nov 01, 2012 at 01:56:36AM +0900, Joonsoo Kim wrote:
>> In current code, after flush_all_zero_pkmaps() is invoked,
>> then re-iterate all pkmaps. It can be optimized if flush_all_zero_pkmaps()
>> return index of first flushed entry. With this index,
>> we can immediately map highmem page to virtual address represented by index.
>> So change return type of flush_all_zero_pkmaps()
>> and return index of first flushed entry.
>>
>> Additionally, update last_pkmap_nr to this index.
>> It is certain that entry which is below this index is occupied by other mapping,
>> therefore updating last_pkmap_nr to this index is reasonable optimization.
>>
>> Cc: Mel Gorman <mel@xxxxxxxxx>
>> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
>> Cc: Minchan Kim <minchan@xxxxxxxxxx>
>> Signed-off-by: Joonsoo Kim <js1304@xxxxxxxxx>
>>
>> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
>> index ef788b5..97ad208 100644
>> --- a/include/linux/highmem.h
>> +++ b/include/linux/highmem.h
>> @@ -32,6 +32,7 @@ static inline void invalidate_kernel_vmap_range(void *vaddr, int size)
>>
>>  #ifdef CONFIG_HIGHMEM
>>  #include <asm/highmem.h>
>> +#define PKMAP_INVALID_INDEX (LAST_PKMAP)
>>
>>  /* declarations for linux/mm/highmem.c */
>>  unsigned int nr_free_highpages(void);
>> diff --git a/mm/highmem.c b/mm/highmem.c
>> index d98b0a9..b365f7b 100644
>> --- a/mm/highmem.c
>> +++ b/mm/highmem.c
>> @@ -106,10 +106,10 @@ struct page *kmap_to_page(void *vaddr)
>>       return virt_to_page(addr);
>>  }
>>
>> -static void flush_all_zero_pkmaps(void)
>> +static unsigned int flush_all_zero_pkmaps(void)
>>  {
>>       int i;
>> -     int need_flush = 0;
>> +     unsigned int index = PKMAP_INVALID_INDEX;
>>
>>       flush_cache_kmaps();
>>
>> @@ -141,10 +141,13 @@ static void flush_all_zero_pkmaps(void)
>>                         &pkmap_page_table[i]);
>>
>>               set_page_address(page, NULL);
>> -             need_flush = 1;
>> +             if (index == PKMAP_INVALID_INDEX)
>> +                     index = i;
>>       }
>> -     if (need_flush)
>> +     if (index != PKMAP_INVALID_INDEX)
>>               flush_tlb_kernel_range(PKMAP_ADDR(0), PKMAP_ADDR(LAST_PKMAP));
>> +
>> +     return index;
>>  }
>>
>>  /**
>> @@ -152,14 +155,19 @@ static void flush_all_zero_pkmaps(void)
>>   */
>>  void kmap_flush_unused(void)
>>  {
>> +     unsigned int index;
>> +
>>       lock_kmap();
>> -     flush_all_zero_pkmaps();
>> +     index = flush_all_zero_pkmaps();
>> +     if (index != PKMAP_INVALID_INDEX && (index < last_pkmap_nr))
>> +             last_pkmap_nr = index;
>
> I don't know how kmap_flush_unused is really fast path so how my nitpick
> is effective. Anyway,
> What problem happens if we do following as?
>
> lock()
> index = flush_all_zero_pkmaps();
> if (index != PKMAP_INVALID_INDEX)
>         last_pkmap_nr = index;
> unlock();
>
> Normally, last_pkmap_nr is increased with searching empty slot in
> map_new_virtual. So I expect return value of flush_all_zero_pkmaps
> in kmap_flush_unused normally become either less than last_pkmap_nr
> or last_pkmap_nr + 1.

There is a case that return value of kmap_flush_unused() is larger
than last_pkmap_nr.
Look at the following example.

Assume last_pkmap = 20 and index 1-9, 11-19 is kmapped. 10 is kunmapped.

do kmap_flush_unused() => flush index 10 => last_pkmap = 10;
do kunmap() with index 17
do kmap_flush_unused() => flush index 17

So, little dirty implementation is needed.

Thanks.
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