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/20221003144832.2906610-1-bgeffon@google.com
Signed-off-by: Brian Geffon <bgeffon@google.com>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Rom Lemarchand <romlem@google.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 43eeef2..2ba5c98 100644 (file)
--- a/drivers/block/zram/zram_drv.c
+++ b/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);