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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