linux-kernel - Re: [PATCH v2 4/5] mm, highmem: makes flush_all_zero_pkmaps() return index of first flushed entry

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Date:	Tue, 13 Nov 2012 09:30:57 +0900
From:	JoonSoo Kim <js1304@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Mel Gorman <mel@....ul.ie>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH v2 4/5] mm, highmem: makes flush_all_zero_pkmaps() return
 index of first flushed entry

2012/11/3 Minchan Kim <minchan@...nel.org>:
> Hi Joonsoo,
>
> On Sat, Nov 03, 2012 at 04:07:25AM +0900, JoonSoo Kim wrote:
>> Hello, Minchan.
>>
>> 2012/11/1 Minchan Kim <minchan@...nel.org>:
>> > 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@....ul.ie>
>> >> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>> >> Cc: Minchan Kim <minchan@...nel.org>
>> >> Signed-off-by: Joonsoo Kim <js1304@...il.com>
>> >>
>> >> 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.

>> 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.
>
> --
> Kind Regards,
> Minchan Kim
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