Patch "mmc: remove bondage between REQ_META and reliable write" has been added to the 3.14-stable tree

This is a note to let you know that I've just added the patch titled

mmc: remove bondage between REQ_META and reliable write

to the 3.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mmc-remove-bondage-between-req_meta-and-reliable-write.patch
and it can be found in the queue-3.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Luca Porzio <[email protected]>
Date: Fri, 6 Nov 2015 15:12:26 +0000
Subject: mmc: remove bondage between REQ_META and reliable write

From: Luca Porzio <[email protected]>

commit d3df0465db00cf4ed9f90d0bfc3b827d32b9c796 upstream.

Anytime a write operation is performed with Reliable Write flag enabled, the eMMC device is enforced to bypass the cache and do a write to the underling NVM device by Jedec specification; this causes a performance penalty since write operations can't be optimized by the device cache.

In our tests, we replayed a typical mobile daily trace pattern and found ~9% overall time reduction in trace replay by using this patch. Also the write ops within 4KB~64KB chunk size range get a 40~60% performance improvement by using the patch (as this range of write chunks are the ones affected by REQ_META).

This patch has been discussed in the Mobile & Embedded Linux Storage Forum and it's the results of feedbacks from many people. We also checked with fsdevl and f2fs mailing list developers that this change in the usage of REQ_META is not affecting FS behavior and we got positive feedbacks. Reporting here the feedbacks:
http://comments.gmane.org/gmane.linux.file-systems/97219
http://thread.gmane.org/gmane.linux.file-systems.f2fs/3178/focus=3183

---
 drivers/mmc/card/block.c |   11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

@@ -62,8 +62,7 @@ MODULE_ALIAS("mmc:block");
 #define MMC_SANITIZE_REQ_TIMEOUT 240000
 #define MMC_EXTRACT_INDEX_FROM_ARG(x) ((x & 0x00FF0000) >> 16)
 
-#define mmc_req_rel_wr(req)	(((req->cmd_flags & REQ_FUA) || \
-				  (req->cmd_flags & REQ_META)) && \
+#define mmc_req_rel_wr(req)	((req->cmd_flags & REQ_FUA) && \
 				  (rq_data_dir(req) == WRITE))
 #define PACKED_CMD_VER	0x01
 #define PACKED_CMD_WR	0x02

@@ -1328,13 +1327,9 @@ static void mmc_blk_rw_rq_prep(struct mm
 
 	/*
 	 * Reliable writes are used to implement Forced Unit Access and
-	 * REQ_META accesses, and are supported only on MMCs.
-	 *
-	 * XXX: this really needs a good explanation of why REQ_META
-	 * is treated special.
+	 * are supported only on MMCs.
 	 */
-	bool do_rel_wr = ((req->cmd_flags & REQ_FUA) ||
-			  (req->cmd_flags & REQ_META)) &&
+	bool do_rel_wr = (req->cmd_flags & REQ_FUA) &&
 		(rq_data_dir(req) == WRITE) &&
 		(md->flags & MMC_BLK_REL_WR);
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.14/mmc-remove-bondage-between-req_meta-and-reliable-write.patch