IS_ENABLED() mainly for CONFIG_* symbols. ZRAM_PARTIAL_IO is a macro
defined as 1 on non-4K builds, so IS_ENABLED(ZRAM_PARTIAL_IO) becomes
IS_ENABLED(1) and evaluates false.

Replace that check with PAGE_SIZE == 4096 and fold is_partial_io() into
one helper so partial-I/O policy stays consistent. PAGE_SIZE is a
build-time constant, so the PAGE_SIZE == 4096 checks fold away on the
configurations where partial I/O is supported.

Tested-on: Raspberry Pi 5 (BCM2712, 4 KiB and 16 KiB page kernels)

Signed-off-by: Jianyue Wu <wujianyue000@gmail.com>
---
To: Minchan Kim <minchan@kernel.org>
To: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: linux-kernel@vger.kernel.org
Cc: linux-block@vger.kernel.org

zram: fix partial I/O gating on non-4K PAGE_SIZE

On PAGE_SIZE > 4K, zram writeback can use sub-page bvec I/O. The
synchronous read_from_bdev() path is used for that case.

v1 used IS_ENABLED(ZRAM_PARTIAL_IO) where ZRAM_PARTIAL_IO is a local
macro defined as 1, so the check expands to IS_ENABLED(1) and is always
false. The WARN_ON_ONCE(!IS_ENABLED(...)) guard then rejects the sync
path with -EIO.

Replace that check with PAGE_SIZE == 4096 and fold is_partial_io() into
one helper so partial-I/O policy stays consistent. PAGE_SIZE is a
build-time constant, so the PAGE_SIZE == 4096 checks fold away on the
configurations where partial I/O is supported.

Testing (Raspberry Pi 5, BCM2712, rpi-6.12.y):

16 KiB kernel (6.12.92-v8-16k+):
- full-page I/O: PASS
- sub-page I/O: PASS
- writeback-backed read: PASS (bd_reads=100)
- no zram WARNING in dmesg

4 KiB kernel (6.12.92-v8-4k+):
- full-page I/O: PASS
- sub-page I/O: PASS (regression; partial path not used by design)
- writeback-backed read: PASS (bd_reads=161)
- no WARN_ON_ONCE(PAGE_SIZE == 4096) in dmesg

Writeback tests use a loop block device as backing_dev. On 4 KiB
builds partial-path success is not required by design because
is_partial_io() is always false when PAGE_SIZE == 4096.
---
Changes in v2:
- Use PAGE_SIZE == 4096 for the read_from_bdev() guard.
- Fold is_partial_io() into one helper.
- Expand commit message with root cause, impact, and build-time note.
- Add Raspberry Pi 5 validation on 4 KiB and 16 KiB kernels.
- Link to v1: https://lore.kernel.org/r/20260531-zram-fix-partial-io-config-check-on-akpm-v1-1-eb085d98faea@gmail.com

Jianyue Wu (1):
  zram: fix partial I/O gating on non-4K PAGE_SIZE
---
 drivers/block/zram/zram_drv.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 6e1330ce4bc1..ddea09afd8dd 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -216,18 +216,12 @@ static bool zram_can_store_page(struct zram *zram)
 	return !zram->limit_pages || alloced_pages <= zram->limit_pages;
 }
 
-#if PAGE_SIZE != 4096
 static inline bool is_partial_io(struct bio_vec *bvec)
 {
+	if (PAGE_SIZE == 4096)
+		return false;
 	return bvec->bv_len != PAGE_SIZE;
 }
-#define ZRAM_PARTIAL_IO		1
-#else
-static inline bool is_partial_io(struct bio_vec *bvec)
-{
-	return false;
-}
-#endif
 
 #if defined CONFIG_ZRAM_WRITEBACK || defined CONFIG_ZRAM_MULTI_COMP
 struct zram_pp_slot {
@@ -1510,7 +1504,8 @@ static int read_from_bdev(struct zram *zram, struct page *page, u32 index,
 {
 	atomic64_inc(&zram->stats.bd_reads);
 	if (!parent) {
-		if (WARN_ON_ONCE(!IS_ENABLED(ZRAM_PARTIAL_IO)))
+		/* Sub-page I/O only exists on non-4K PAGE_SIZE builds. */
+		if (WARN_ON_ONCE(PAGE_SIZE == 4096))
 			return -EIO;
 		return read_from_bdev_sync(zram, page, index, blk_idx);
 	}

---
base-commit: 404fb4f38e8f38469dfff4df0205c9d18eeb1f57
change-id: 20260531-zram-fix-partial-io-config-check-on-akpm-c62b972416f8

Best regards,
-- 
Jianyue Wu <wujianyue000@gmail.com>