linux-kernel - [PATCHv8 2/8] zsmalloc: add documentation

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Date:	Thu,  4 Apr 2013 17:10:40 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>,
	Hugh Dickens <hughd@...gle.com>,
	Paul Mackerras <paulus@...ba.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: [PATCHv8 2/8] zsmalloc: add documentation

This patch adds a documentation file for zsmalloc at
Documentation/vm/zsmalloc.txt

Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>
---
 Documentation/vm/zsmalloc.txt | 68 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 68 insertions(+)
 create mode 100644 Documentation/vm/zsmalloc.txt

diff --git a/Documentation/vm/zsmalloc.txt b/Documentation/vm/zsmalloc.txt
new file mode 100644
index 0000000..85aa617
--- /dev/null
+++ b/Documentation/vm/zsmalloc.txt
@@ -0,0 +1,68 @@
+zsmalloc Memory Allocator
+
+Overview
+
+zmalloc a new slab-based memory allocator,
+zsmalloc, for storing compressed pages.  It is designed for
+low fragmentation and high allocation success rate on
+large object, but <= PAGE_SIZE allocations.
+
+zsmalloc differs from the kernel slab allocator in two primary
+ways to achieve these design goals.
+
+zsmalloc never requires high order page allocations to back
+slabs, or "size classes" in zsmalloc terms. Instead it allows
+multiple single-order pages to be stitched together into a
+"zspage" which backs the slab.  This allows for higher allocation
+success rate under memory pressure.
+
+Also, zsmalloc allows objects to span page boundaries within the
+zspage.  This allows for lower fragmentation than could be had
+with the kernel slab allocator for objects between PAGE_SIZE/2
+and PAGE_SIZE.  With the kernel slab allocator, if a page compresses
+to 60% of it original size, the memory savings gained through
+compression is lost in fragmentation because another object of
+the same size can't be stored in the leftover space.
+
+This ability to span pages results in zsmalloc allocations not being
+directly addressable by the user.  The user is given an
+non-dereferencable handle in response to an allocation request.
+That handle must be mapped, using zs_map_object(), which returns
+a pointer to the mapped region that can be used.  The mapping is
+necessary since the object data may reside in two different
+noncontigious pages.
+
+For 32-bit systems, zsmalloc has the added benefit of being
+able to back slabs with HIGHMEM pages, something not possible
+with the kernel slab allocators (SLAB or SLUB).
+
+Usage:
+
+#include <linux/zsmalloc.h>
+
+/* create a new pool */
+struct zs_pool *pool = zs_create_pool("mypool", GFP_KERNEL);
+
+/* allocate a 256 byte object */
+unsigned long handle = zs_malloc(pool, 256);
+
+/*
+ * Map the object to get a dereferenceable pointer in "read-write mode"
+ * (see zsmalloc.h for additional modes)
+ */
+void *ptr = zs_map_object(pool, handle, ZS_MM_RW);
+
+/* do something with ptr */
+
+/*
+ * Unmap the object when done dealing with it. You should try to
+ * minimize the time for which the object is mapped since preemption
+ * is disabled during the mapped period.
+ */
+zs_unmap_object(pool, handle);
+
+/* free the object */
+zs_free(pool, handle);
+
+/* destroy the pool */
+zs_destroy_pool(pool); 
-- 
1.8.2

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