zsmalloc: introduce SG-list based object read API

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.

[senozhatsky@chromium.org: make zs_obj_read_sg_begin() return void, per Yosry]
  Link: https://lkml.kernel.org/r/20260117024900.792237-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20260113034645.2729998-1-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 67 insertions, 0 deletions

diff --git a/include/linux/zsmalloc.h b/include/linux/zsmalloc.h
index 5565c3171007..478410c880b1 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);
+void 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 cc3d9501ae21..dccb88d52c07 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>
@@ -1141,6 +1142,68 @@ void zs_obj_read_end(struct zs_pool *pool, unsigned long handle,
 }
 EXPORT_SYMBOL_GPL(zs_obj_read_end);
 
+void 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);
+	}
+}
+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)
 {