+ fat-improve-fat_sync_bhs-write-submission.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: fat: improve fat_sync_bhs() write submission
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     fat-improve-fat_sync_bhs-write-submission.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fat-improve-fat_sync_bhs-write-submission.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br>
Subject: fat: improve fat_sync_bhs() write submission
Date: Mon, 26 Oct 2026 13:39:41 -0300

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

Link: https://lore.kernel.org/20261026163941.495833-3-matheus.aalmeida@inf.ufrgs.br
Signed-off-by: Matheus Alves de Almeida <matheus.aalmeida@inf.ufrgs.br>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
---

 fs/fat/misc.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/fs/fat/misc.c~fat-improve-fat_sync_bhs-write-submission
+++ a/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
@@ -353,9 +354,14 @@ 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]);
_

Patches currently in -mm which might be from matheus.aalmeida@inf.ufrgs.br are

fat-preallocate-space-before-expanding-in-fat_cont_expand.patch
fat-hold-truncate_lock-in-fat_write_failed.patch
fat-batch-synchronous-mirror-writes.patch
fat-improve-fat_sync_bhs-write-submission.patch