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.
> 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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