2012/11/3 Minchan Kim [off-list ref]:

Hi Joonsoo,

On Sat, Nov 03, 2012 at 04:07:25AM +0900, JoonSoo Kim wrote:

quoted

Hello, Minchan.

2012/11/1 Minchan Kim [off-list ref]:

quoted

On Thu, Nov 01, 2012 at 01:56:36AM +0900, Joonsoo Kim wrote:

quoted

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 <redacted>
Cc: Peter Zijlstra <redacted>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Joonsoo Kim <redacted>

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.

quoted

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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