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

[Minchan Kim <minchan@kernel.org>]

On Tue, Nov 13, 2012 at 09:30:57AM +0900, JoonSoo Kim wrote:
> 2012/11/3 Minchan Kim <minchan@kernel.org>:
> > Hi Joonsoo,
> >
> > On Sat, Nov 03, 2012 at 04:07:25AM +0900, JoonSoo Kim wrote:
> >> Hello, Minchan.
> >>
> >> 2012/11/1 Minchan Kim <minchan@kernel.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@csn.ul.ie>
> >> >> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> >> >> Cc: Minchan Kim <minchan@kernel.org>
> >> >> Signed-off-by: Joonsoo Kim <js1304@gmail.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.

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?
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.


-- 
Kind Regards,
Minchan Kim

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