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>

1 files changed, 4 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);