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:	Sat, 3 Nov 2012 07:42:36 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	JoonSoo Kim <js1304@...il.com>
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

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.

> 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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