zram: always compile read_from_bdev_sync

Patch series "zram I/O path cleanups and fixups", v3.

This series cleans up the zram I/O path, and fixes the handling of
synchronous I/O to the underlying device in the writeback_store function
or for > 4K PAGE_SIZE systems.

The fixes are at the end, as I could not fully reason about them being
safe before untangling the callchain.


This patch (of 17):

read_from_bdev_sync is currently only compiled for non-4k PAGE_SIZE, which
means it won't be built with the most common configurations.

Replace the ifdef with a check for the PAGE_SIZE in an if instead.  The
check uses an extra symbol and IS_ENABLED to allow the compiler to
eliminate the dead code, leading to the same generated code size:

   text	   data	    bss	    dec	    hex	filename
  16709	   1428	     12	  18149	   46e5	drivers/block/zram/zram_drv.o.old
  16709	   1428	     12	  18149	   46e5	drivers/block/zram/zram_drv.o.new

Link: https://lkml.kernel.org/r/20230411171459.567614-1-hch@lst.de
Link: https://lkml.kernel.org/r/20230411171459.567614-2-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -148,6 +148,7 @@ static inline bool is_partial_io(struct bio_vec *bvec)
 {
 	return bvec->bv_len != PAGE_SIZE;
 }
+#define ZRAM_PARTIAL_IO		1
 #else
 static inline bool is_partial_io(struct bio_vec *bvec)
 {
@@ -833,7 +834,6 @@ struct zram_work {
 	struct bio_vec bvec;
 };
 
-#if PAGE_SIZE != 4096
 static void zram_sync_read(struct work_struct *work)
 {
 	struct zram_work *zw = container_of(work, struct zram_work, work);
@@ -866,23 +866,17 @@ static int read_from_bdev_sync(struct zram *zram, struct bio_vec *bvec,
 
 	return 1;
 }
-#else
-static int read_from_bdev_sync(struct zram *zram, struct bio_vec *bvec,
-				unsigned long entry, struct bio *bio)
-{
-	WARN_ON(1);
-	return -EIO;
-}
-#endif
 
 static int read_from_bdev(struct zram *zram, struct bio_vec *bvec,
 			unsigned long entry, struct bio *parent, bool sync)
 {
 	atomic64_inc(&zram->stats.bd_reads);
-	if (sync)
+	if (sync) {
+		if (WARN_ON_ONCE(!IS_ENABLED(ZRAM_PARTIAL_IO)))
+			return -EIO;
 		return read_from_bdev_sync(zram, bvec, entry, parent);
-	else
-		return read_from_bdev_async(zram, bvec, entry, parent);
+	}
+	return read_from_bdev_async(zram, bvec, entry, parent);
 }
 #else
 static inline void reset_bdev(struct zram *zram) {};