[PATCH] zsmalloc: introduce SG-list based object read API

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Currently, zsmalloc performs address linearization on read
(which sometimes requires memcpy() to a local buffer).
Not all zsmalloc users need a linear address. For example,
Crypto API supports SG-list, performing linearization under
the hood, if needed. In addition, some compressors can have
native SG-list support, completely avoiding the linearization
step.

Provide an SG-list based zsmalloc read API:
- zs_obj_read_sg_begin()
- zs_obj_read_sg_end()

This API allows callers to obtain an SG representation
of the object (one entry for objects that are contained
in a single page and two entries for spanning objects),
avoiding the need for a bounce buffer and memcpy.

Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
---
 include/linux/zsmalloc.h |  4 +++
 mm/zsmalloc.c            | 65 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 5565c3171007..11e614663dd3 100644
--- a/include/linux/zsmalloc.h
+++ b/include/linux/zsmalloc.h
@@ -22,6 +22,7 @@ struct zs_pool_stats {
 };
 
 struct zs_pool;
+struct scatterlist;
 
 struct zs_pool *zs_create_pool(const char *name);
 void zs_destroy_pool(struct zs_pool *pool);
@@ -43,6 +44,9 @@ void *zs_obj_read_begin(struct zs_pool *pool, unsigned long handle,
 			size_t mem_len, void *local_copy);
 void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
 		     size_t mem_len, void *handle_mem);
+int zs_obj_read_sg_begin(struct zs_pool *pool, unsigned long handle,
+		   struct scatterlist *sg, size_t mem_len);
+void zs_obj_read_sg_end(struct zs_pool *pool, unsigned long handle);
 void zs_obj_write(struct zs_pool *pool, unsigned long handle,
 		  void *handle_mem, size_t mem_len);
 
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 16d5587a052a..5abb8bc0956a 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -30,6 +30,7 @@
 #include <linux/highmem.h>
 #include <linux/string.h>
 #include <linux/slab.h>
+#include <linux/scatterlist.h>
 #include <linux/spinlock.h>
 #include <linux/sprintf.h>
 #include <linux/shrinker.h>
@@ -1146,6 +1147,70 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
 }
 EXPORT_SYMBOL_GPL(zs_obj_read_end);
 
+int zs_obj_read_sg_begin(struct zs_pool *pool, unsigned long handle,
+			 struct scatterlist *sg, size_t mem_len)
+{
+	struct zspage *zspage;
+	struct zpdesc *zpdesc;
+	unsigned long obj, off;
+	unsigned int obj_idx;
+	struct size_class *class;
+
+	/* Guarantee we can get zspage from handle safely */
+	read_lock(&pool->lock);
+	obj = handle_to_obj(handle);
+	obj_to_location(obj, &zpdesc, &obj_idx);
+	zspage = get_zspage(zpdesc);
+
+	/* Make sure migration doesn't move any pages in this zspage */
+	zspage_read_lock(zspage);
+	read_unlock(&pool->lock);
+
+	class = zspage_class(pool, zspage);
+	off = offset_in_page(class->size * obj_idx);
+
+	if (!ZsHugePage(zspage))
+		off += ZS_HANDLE_SIZE;
+
+	if (off + mem_len <= PAGE_SIZE) {
+		/* this object is contained entirely within a page */
+		sg_init_table(sg, 1);
+		sg_set_page(sg, zpdesc_page(zpdesc), mem_len, off);
+	} else {
+		size_t sizes[2];
+
+		/* this object spans two pages */
+		sizes[0] = PAGE_SIZE - off;
+		sizes[1] = mem_len - sizes[0];
+
+		sg_init_table(sg, 2);
+		sg_set_page(sg, zpdesc_page(zpdesc), sizes[0], off);
+
+		zpdesc = get_next_zpdesc(zpdesc);
+		sg = sg_next(sg);
+
+		sg_set_page(sg, zpdesc_page(zpdesc), sizes[1], 0);
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(zs_obj_read_sg_begin);
+
+void zs_obj_read_sg_end(struct zs_pool *pool, unsigned long handle)
+{
+	struct zspage *zspage;
+	struct zpdesc *zpdesc;
+	unsigned long obj;
+	unsigned int obj_idx;
+
+	obj = handle_to_obj(handle);
+	obj_to_location(obj, &zpdesc, &obj_idx);
+	zspage = get_zspage(zpdesc);
+
+	zspage_read_unlock(zspage);
+}
+EXPORT_SYMBOL_GPL(zs_obj_read_sg_end);
+
 void zs_obj_write(struct zs_pool *pool, unsigned long handle,
 		  void *handle_mem, size_t mem_len)
 {
-- 
2.52.0.457.g6b5491de43-goog