zswap: only save zswap header when necessary

We waste sizeof(swp_entry_t) for zswap header when using zsmalloc as
zpool driver because zsmalloc doesn't support eviction.

Add zpool_evictable() to detect if zpool is potentially evictable, and
use it in zswap to avoid waste memory for zswap header.

[yuzhao@google.com: The zpool->" prefix is a result of copy & paste]
  Link: http://lkml.kernel.org/r/20180110225626.110330-1-yuzhao@google.com
Link: http://lkml.kernel.org/r/20180110224741.83751-1-yuzhao@google.com
Signed-off-by: Yu Zhao <yuzhao@google.com>
Acked-by: Dan Streetman <ddstreet@ieee.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 35 insertions, 19 deletions

diff --git a/include/linux/zpool.h b/include/linux/zpool.h
index 004ba807df96..7238865e75b0 100644
--- a/include/linux/zpool.h
+++ b/include/linux/zpool.h
@@ -108,4 +108,6 @@ void zpool_register_driver(struct zpool_driver *driver);
 
 int zpool_unregister_driver(struct zpool_driver *driver);
 
+bool zpool_evictable(struct zpool *pool);
+
 #endif
diff --git a/mm/zpool.c b/mm/zpool.c
index fd3ff719c32c..e1e7aa6d1d06 100644
--- a/mm/zpool.c
+++ b/mm/zpool.c
@@ -21,6 +21,7 @@ struct zpool {
         struct zpool_driver *driver;
         void *pool;
         const struct zpool_ops *ops;
+        bool evictable;
 
         struct list_head list;
 };
@@ -142,7 +143,7 @@ EXPORT_SYMBOL(zpool_has_pool);
  *
  * This creates a new zpool of the specified type.  The gfp flags will be
  * used when allocating memory, if the implementation supports it.  If the
- * ops param is NULL, then the created zpool will not be shrinkable.
+ * ops param is NULL, then the created zpool will not be evictable.
  *
  * Implementations must guarantee this to be thread-safe.
  *
@@ -180,6 +181,7 @@ struct zpool *zpool_create_pool(const char *type, const char *name, gfp_t gfp,
         zpool->driver = driver;
         zpool->pool = driver->create(name, gfp, ops, zpool);
         zpool->ops = ops;
+        zpool->evictable = driver->shrink && ops && ops->evict;
 
         if (!zpool->pool) {
                 pr_err("couldn't create %s pool\n", type);
@@ -296,7 +298,8 @@ void zpool_free(struct zpool *zpool, unsigned long handle)
 int zpool_shrink(struct zpool *zpool, unsigned int pages,
                         unsigned int *reclaimed)
 {
-        return zpool->driver->shrink(zpool->pool, pages, reclaimed);
+        return zpool->driver->shrink ?
+               zpool->driver->shrink(zpool->pool, pages, reclaimed) : -EINVAL;
 }
 
 /**
@@ -355,6 +358,24 @@ u64 zpool_get_total_size(struct zpool *zpool)
         return zpool->driver->total_size(zpool->pool);
 }
 
+/**
+ * zpool_evictable() - Test if zpool is potentially evictable
+ * @pool        The zpool to test
+ *
+ * Zpool is only potentially evictable when it's created with struct
+ * zpool_ops.evict and its driver implements struct zpool_driver.shrink.
+ *
+ * However, it doesn't necessarily mean driver will use zpool_ops.evict
+ * in its implementation of zpool_driver.shrink. It could do internal
+ * defragmentation instead.
+ *
+ * Returns: true if potentially evictable; false otherwise.
+ */
+bool zpool_evictable(struct zpool *zpool)
+{
+        return zpool->evictable;
+}
+
 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Dan Streetman <ddstreet@ieee.org>");
 MODULE_DESCRIPTION("Common API for compressed memory storage");
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index e136a8e72c48..f797d8b0d820 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -404,12 +404,6 @@ static void zs_zpool_free(void *pool, unsigned long handle)
         zs_free(pool, handle);
 }
 
-static int zs_zpool_shrink(void *pool, unsigned int pages,
-                        unsigned int *reclaimed)
-{
-        return -EINVAL;
-}
-
 static void *zs_zpool_map(void *pool, unsigned long handle,
                         enum zpool_mapmode mm)
 {
@@ -447,7 +441,6 @@ static struct zpool_driver zs_zpool_driver = {
         .destroy =      zs_zpool_destroy,
         .malloc =       zs_zpool_malloc,
         .free =         zs_zpool_free,
-        .shrink =       zs_zpool_shrink,
         .map =          zs_zpool_map,
         .unmap =        zs_zpool_unmap,
         .total_size =   zs_zpool_total_size,
diff --git a/mm/zswap.c b/mm/zswap.c
index 1133b4ceb72e..c004aa4fd3f4 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1001,11 +1001,11 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
         struct zswap_entry *entry, *dupentry;
         struct crypto_comp *tfm;
         int ret;
-        unsigned int dlen = PAGE_SIZE, len;
+        unsigned int hlen, dlen = PAGE_SIZE;
         unsigned long handle, value;
         char *buf;
         u8 *src, *dst;
-        struct zswap_header *zhdr;
+        struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
 
         if (!zswap_enabled || !tree) {
                 ret = -ENODEV;
@@ -1063,8 +1063,8 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
         }
 
         /* store */
-        len = dlen + sizeof(struct zswap_header);
-        ret = zpool_malloc(entry->pool->zpool, len,
+        hlen = zpool_evictable(entry->pool->zpool) ? sizeof(zhdr) : 0;
+        ret = zpool_malloc(entry->pool->zpool, hlen + dlen,
                            __GFP_NORETRY | __GFP_NOWARN | __GFP_KSWAPD_RECLAIM,
                            &handle);
         if (ret == -ENOSPC) {
@@ -1075,10 +1075,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
                 zswap_reject_alloc_fail++;
                 goto put_dstmem;
         }
-        zhdr = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
-        zhdr->swpentry = swp_entry(type, offset);
-        buf = (u8 *)(zhdr + 1);
-        memcpy(buf, dst, dlen);
+        buf = zpool_map_handle(entry->pool->zpool, handle, ZPOOL_MM_RW);
+        memcpy(buf, &zhdr, hlen);
+        memcpy(buf + hlen, dst, dlen);
         zpool_unmap_handle(entry->pool->zpool, handle);
         put_cpu_var(zswap_dstmem);
 
@@ -1149,8 +1148,9 @@ static int zswap_frontswap_load(unsigned type, pgoff_t offset,
 
         /* decompress */
         dlen = PAGE_SIZE;
-        src = (u8 *)zpool_map_handle(entry->pool->zpool, entry->handle,
-                        ZPOOL_MM_RO) + sizeof(struct zswap_header);
+        src = zpool_map_handle(entry->pool->zpool, entry->handle, ZPOOL_MM_RO);
+        if (zpool_evictable(entry->pool->zpool))
+                src += sizeof(struct zswap_header);
         dst = kmap_atomic(page);
         tfm = *get_cpu_ptr(entry->pool->tfm);
         ret = crypto_comp_decompress(tfm, src, entry->length, dst, &dlen);