[PATCH v2 2/2] fat: improve fat_sync_bhs() write submission

[Matheus Alves de Almeida]

fat_sync_bhs() does many individual write_dirty_buffer() calls without
plugging, making batching in the block layer more difficult. It also
does not pass the REQ_SYNC hint for these synchronous writes.

Use a plug around the writes so the block layer can more easily batch
them. Pass the REQ_SYNC hint to write_dirty_buffer().

Benchmarking was done under QEMU/KVM with a virtio-blk device using
cache=directsync. Each result is the arithmetic mean of 10 samples.

start = now_ns();
for (unsigned i = 0; i < count; i++) {
    fallocate(fds[i], FALLOC_FL_KEEP_SIZE, 0, bytes);
}
elapsed = now_ns() - start;

Sync-mounted FAT32 using count=16 files:
File size    Cluster size    Before    After    Improvement
256MiB       512B            13.11s    11.87s   1.10x
256MiB       4096B           1.657s    1.469s   1.13x

Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@xxxxxxxxxxxx>
---
 fs/fat/misc.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/fat/misc.c b/fs/fat/misc.c
index b6a9e2e122cd..e8e2f13254d5 100644
--- a/fs/fat/misc.c
+++ b/fs/fat/misc.c
@@ -9,6 +9,7 @@
 
 #include "fat.h"
 #include <linux/iversion.h>
+#include <linux/blk_plug.h>
 
 /*
  * fat_fs_error reports a file system problem that might indicate fa data
@@ -354,9 +355,14 @@ EXPORT_SYMBOL_GPL(fat_update_time);
 int fat_sync_bhs(struct buffer_head **bhs, int nr_bhs)
 {
 	int i, err = 0;
+	struct blk_plug plug;
+
+	blk_start_plug(&plug);
 
 	for (i = 0; i < nr_bhs; i++)
-		write_dirty_buffer(bhs[i], 0);
+		write_dirty_buffer(bhs[i], REQ_SYNC);
+
+	blk_finish_plug(&plug);
 
 	for (i = 0; i < nr_bhs; i++) {
 		wait_on_buffer(bhs[i]);
-- 
2.43.0