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mm/zsmalloc: support max obj with length of PAGE_SIZE

PATCH New Mahendran Ganesh 1 participant 1 message Nov 21, 2014

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Memory management Zsmalloc compressed slab memory allocator
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Mahendran Ganesh

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PATCH 1/1 mm/zsmalloc: support max obj with length of PAGE_SIZE

2014-11-21 18:32 GMT+08:00 Minchan Kim <[email protected]>:

Quoted 44 lines · Minchan Kim
Quoted 27 lines · Ganesh Mahendran
Quoted 17 lines

Hello

2014-11-21 11:54 GMT+08:00 Minchan Kim <[email protected]>:

Quoted 8 lines · Mahendran Ganesh
Quoted 7 lines

ZS_SIZE_CLASSES is calc by:
((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1)

So when i is in [0, ZS_SIZE_CLASSES - 1), the size:
size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA
will not be greater than ZS_MAX_ALLOC_SIZE

Quoted 6 lines
Quoted 1 line

This patch removes the unnecessary check.

It depends on ZS_MIN_ALLOC_SIZE.
For example, we would change min to 8 but MAX is still 4096. ZS_SIZE_CLASSES is (4096 - 8) / 16 + 1 = 256 so 8 + 255 * 16 = 4088, which exceeds the max.

Quoted 8 lines

Here, 4088 is less than MAX(4096).

ZS_SIZE_CLASSES = (MAX - MIN) / Delta + 1
So, I think the value of
MIN + (ZS_SIZE_CLASSES - 1) * Delta =
MIN + ((MAX - MIN) / Delta) * Delta =
MAX
will not exceed the MAX

Quoted 6 lines

You're right. It was complext math for me.
I should go back to elementary school.

Thanks!

Acked-by Minchan Kim <[email protected]>

I catch a nasty cold but above my poor math makes me think more. ZS_SIZE_CLASSES is broken. In above my example, current code cannot allocate 4096 size class so we should correct ZS_SIZE_CLASSES at first.

zs_size_classes = zs_max - zs_min / delta + 1;
if ((zs_max - zs_min) % delta)

        zs_size_classes += 1;
Quoted 7 lines

Then, we need to code piece you removed.
As well, we need to fix below.

- area->vm_buf = (char *)__get_free_page(GFP_KERNEL);
+ area->vm_buf = kmalloc(ZS_MAX_ALLOC_SIZE);

Hope I am sane in this time :(

how about something like this?

In zsmalloc, if ZS_MIN_ALLOC_SIZE is less than ZS_SIZE_CLASS_DELTA. Max obj size (ZS_MIN_ALLOC_SIZE + ZS_SIZE_CLASSES * ZS_SIZE_CLASS_DELTA) will be less than ZS_MAX_ALLOC_SIZE(page size).
And in zs_malloc(), we thought we can put an obj(len == PAGE_SIZE) into zsmalloc. But actually we can not. This will make user confused.

This patch takes Minchan Kim's suggestion.

    https://lkml.org/lkml/2014/11/21/172

When (ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) % ZS_SIZE_CLASS_DELTA) != 0, we add increase sz_size_class by 1 to make the size of max obj equal to ZS_MAX_ALLOC_SIZE(PAGE_SIZE).

Signed-off-by Mahendran Ganesh <[email protected]>
---
 mm/zsmalloc.c |   22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)
mm/zsmalloc.c +17 -5
index 810eda1..bcea72d 100644
@@ -155,8 +155,6 @@
  *  (reason above)
  */
 #define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> 8)
-#define ZS_SIZE_CLASSES		((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / \
-					ZS_SIZE_CLASS_DELTA + 1)
 
 /*
  * We do not maintain any list for completely empty or full pages

@@ -214,7 +212,8 @@ struct link_free {
 };
 
 struct zs_pool {
-	struct size_class *size_class[ZS_SIZE_CLASSES];
+	struct size_class **size_class;
+	int nr_size_classes;
 
 	gfp_t flags;	/* allocation flags used when growing pool */
 	atomic_long_t pages_allocated;

@@ -956,17 +955,28 @@ struct zs_pool *zs_create_pool(gfp_t flags)
 {
 	int i, ovhd_size;
 	struct zs_pool *pool;
+	int nr;
 
 	ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
 	pool = kzalloc(ovhd_size, GFP_KERNEL);
 	if (!pool)
 		return NULL;
 
+	nr = (ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) / ZS_SIZE_CLASS_DELTA + 1;
+	if ((ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE) % ZS_SIZE_CLASS_DELTA)
+		nr += 1;
+
+	pool->size_class = kcalloc(nr, sizeof(struct size_class *),
+			GFP_KERNEL);
+	if (!pool->size_class)
+		goto err;
+	pool->nr_size_classes = nr;
+
 	/*
 	 * Iterate reversly, because, size of size_class that we want to use
 	 * for merging should be larger or equal to current size.
 	 */
-	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
+	for (i = pool->nr_size_classes - 1; i >= 0; i--) {
 		int size;
 		int pages_per_zspage;
 		struct size_class *class;

@@ -1020,7 +1030,7 @@ void zs_destroy_pool(struct zs_pool *pool)
 {
 	int i;
 
-	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+	for (i = 0; i < pool->nr_size_classes; i++) {
 		int fg;
 		struct size_class *class = pool->size_class[i];
 

@@ -1038,6 +1048,8 @@ void zs_destroy_pool(struct zs_pool *pool)
 		}
 		kfree(class);
 	}
+
+	kfree(pool->size_class);
 	kfree(pool);
 }
 EXPORT_SYMBOL_GPL(zs_destroy_pool);
Signature · 7 lines
1.7.9.5

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