linux-kernel - [PATCH v2 3/3] zram: Shrink zram_table

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Date: Thu, 20 Jun 2024 17:28:52 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Minchan Kim <minchan@...nel.org>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Jens Axboe <axboe@...nel.dk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v2 3/3] zram: Shrink zram_table_entry::flags.

The zram_table_entry::flags member is of type long and uses 8 bytes on a
64bit architecture. With a PAGE_SIZE of 256KiB we have PAGE_SHIFT of 18
which in turn leads to __NR_ZRAM_PAGEFLAGS = 27. This still fits in an
ordinary integer.
By reducing it the size of `flags' to four bytes, the size of the struct
goes back to 16 bytes. The padding between the lock and ac_time (if
enabled) is also gone.

Make zram_table_entry::flags an unsigned int and update the build test
to reflect the change.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 drivers/block/zram/zram_drv.c | 3 ++-
 drivers/block/zram/zram_drv.h | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 659966e00c300..a35d4bd2e60ef 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -2412,9 +2412,10 @@ static void destroy_devices(void)
 
 static int __init zram_init(void)
 {
+	struct zram_table_entry zram_te;
 	int ret;
 
-	BUILD_BUG_ON(__NR_ZRAM_PAGEFLAGS > BITS_PER_LONG);
+	BUILD_BUG_ON(__NR_ZRAM_PAGEFLAGS > sizeof(zram_te.flags) * 8);
 
 	ret = cpuhp_setup_state_multi(CPUHP_ZCOMP_PREPARE, "block/zram:prepare",
 				      zcomp_cpu_up_prepare, zcomp_cpu_dead);
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 262fa960a0783..531cefc666682 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -66,7 +66,7 @@ struct zram_table_entry {
 		unsigned long handle;
 		unsigned long element;
 	};
-	unsigned long flags;
+	unsigned int flags;
 	spinlock_t lock;
 #ifdef CONFIG_ZRAM_TRACK_ENTRY_ACTIME
 	ktime_t ac_time;
-- 
2.45.2