zram: always expose rw_page

Currently zram will adjust its fops to a version which does not contain
rw_page when a backing device has been assigned.  This is done to prevent
upper layers from assuming a synchronous operation when a page may have
been written back.  This forces every operation through bio which has
overhead associated with bio_alloc/frees.

The code can be simplified to always expose an rw_page method and only in
the rare event that a page is written back we instead will return
-EOPNOTSUPP forcing the upper layer to fallback to bio.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Brian Geffon <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Nitin Gupta <[email protected]>
Cc: Rom Lemarchand <[email protected]>
Cc: Suleiman Souhlal <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: bgaff

drivers/block/zram/zram_drv.c

@@ -52,9 +52,6 @@ static unsigned int num_devices = 1;
 static size_t huge_class_size;
 
 static const struct block_device_operations zram_devops;
-#ifdef CONFIG_ZRAM_WRITEBACK
-static const struct block_device_operations zram_wb_devops;
-#endif
 
 static void zram_free_page(struct zram *zram, size_t index);
 static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
@@ -546,17 +543,6 @@ static ssize_t backing_dev_store(struct device *dev,
 	zram->backing_dev = backing_dev;
 	zram->bitmap = bitmap;
 	zram->nr_pages = nr_pages;
-	/*
-	 * With writeback feature, zram does asynchronous IO so it's no longer
-	 * synchronous device so let's remove synchronous io flag. Othewise,
-	 * upper layer(e.g., swap) could wait IO completion rather than
-	 * (submit and return), which will cause system sluggish.
-	 * Furthermore, when the IO function returns(e.g., swap_readpage),
-	 * upper layer expects IO was done so it could deallocate the page
-	 * freely but in fact, IO is going on so finally could cause
-	 * use-after-free when the IO is really done.
-	 */
-	zram->disk->fops = &zram_wb_devops;
 	up_write(&zram->init_lock);
 
 	pr_info("setup backing device %s\n", file_name);
@@ -1270,6 +1256,9 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
 		struct bio_vec bvec;
 
 		zram_slot_unlock(zram, index);
+		/* A null bio means rw_page was used, we must fallback to bio */
+		if (!bio)
+			return -EOPNOTSUPP;
 
 		bvec.bv_page = page;
 		bvec.bv_len = PAGE_SIZE;
@@ -1856,15 +1845,6 @@ static const struct block_device_operations zram_devops = {
 	.owner = THIS_MODULE
 };
 
-#ifdef CONFIG_ZRAM_WRITEBACK
-static const struct block_device_operations zram_wb_devops = {
-	.open = zram_open,
-	.submit_bio = zram_submit_bio,
-	.swap_slot_free_notify = zram_slot_free_notify,
-	.owner = THIS_MODULE
-};
-#endif
-
 static DEVICE_ATTR_WO(compact);
 static DEVICE_ATTR_RW(disksize);
 static DEVICE_ATTR_RO(initstate);