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

[JoonSoo Kim]

2012/11/13 Minchan Kim <minchan@xxxxxxxxxx>:
> On Tue, Nov 13, 2012 at 09:30:57AM +0900, JoonSoo Kim wrote:
>> 2012/11/3 Minchan Kim <minchan@xxxxxxxxxx>:
>> > Hi Joonsoo,
>> >
>> > On Sat, Nov 03, 2012 at 04:07:25AM +0900, JoonSoo Kim wrote:
>> >> 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.
>> >
>> > I see but why it's problem? kmap_flush_unused returns larger value than
>> > last_pkmap_nr means that there is no free slot at below the value.
>> > So unconditional last_pkmap_nr update is vaild.
>>
>> I think that this is not true.
>> Look at the slightly different example.
>>
>> Assume last_pkmap = 20 and index 1-9, 12-19 is kmapped. 10, 11 is kunmapped.
>>
>> do kmap_flush_unused() => flush index 10,11 => last_pkmap = 10;
>> do kunmap() with index 17
>> do kmap_flush_unused() => flush index 17 => last_pkmap = 17?
>>
>> In this case, unconditional last_pkmap_nr update skip one kunmapped index.
>> So, conditional update is needed.
>
> Thanks for pouinting out, Joonsoo.
> You're right. I misunderstood your flush_all_zero_pkmaps change.
> As your change, flush_all_zero_pkmaps returns first *flushed* free slot index.
> What's the benefit returning flushed flushed free slot index rather than free slot index?

If flush_all_zero_pkmaps() return free slot index rather than first
flushed free slot,
we need another comparison like as 'if pkmap_count[i] == 0' and
need another local variable for determining whether flush is occurred or not.
I want to minimize these overhead and churning of the code, although
they are negligible.

> I think flush_all_zero_pkmaps should return first free slot because customer of
> flush_all_zero_pkmaps doesn't care whether it's just flushed or not.
> What he want is just free or not. In such case, we can remove above check and it makes
> flusha_all_zero_pkmaps more intuitive.

Yes, it is more intuitive, but as I mentioned above, it need another comparison,
so with that, a benefit which prevent to re-iterate when there is no
free slot, may be disappeared.
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