md: Remove flush handling

For flush request, md has a special flush handling to merge concurrent
flush request into single one, however, the whole mechanism is based on
a disk level spin_lock 'mddev->lock'. And fsync can be called quite
often in some user cases, for consequence, spin lock from IO fast path can
cause performance degradation.

Fortunately, the block layer already has flush handling to merge
concurrent flush request, and it only acquires hctx level spin lock. (see
details in blk-flush.c)

This patch removes the flush handling in md, and converts to use general
block layer flush handling in underlying disks.

Flush test for 4 nvme raid10:
start 128 threads to do fsync 100000 times, on arm64, see how long it
takes.

Test script:
void* thread_func(void* arg) {
    int fd = *(int*)arg;
    for (int i = 0; i < FSYNC_COUNT; i++) {
        fsync(fd);
    }
    return NULL;
}

int main() {
    int fd = open("/dev/md0", O_RDWR);
    if (fd < 0) {
        perror("open");
        exit(1);
    }

    pthread_t threads[THREADS];
    struct timeval start, end;

    gettimeofday(&start, NULL);

    for (int i = 0; i < THREADS; i++) {
        pthread_create(&threads[i], NULL, thread_func, &fd);
    }

    for (int i = 0; i < THREADS; i++) {
        pthread_join(threads[i], NULL);
    }

    gettimeofday(&end, NULL);

    close(fd);

    long long elapsed = (end.tv_sec - start.tv_sec) * 1000000LL + (end.tv_usec - start.tv_usec);
    printf("Elapsed time: %lld microseconds\n", elapsed);

    return 0;
}

Test result: about 10 times faster:
Before this patch: 50943374 microseconds
After this patch:  5096347  microseconds

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240827110616.3860190-1-yukuai1@huaweicloud.com
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md.h

@@ -571,16 +571,6 @@ struct mddev {
 						   */
 	struct bio_set			io_clone_set;
 
-	/* Generic flush handling.
-	 * The last to finish preflush schedules a worker to submit
-	 * the rest of the request (without the REQ_PREFLUSH flag).
-	 */
-	struct bio *flush_bio;
-	atomic_t flush_pending;
-	ktime_t start_flush, prev_flush_start; /* prev_flush_start is when the previous completed
-						* flush was started.
-						*/
-	struct work_struct flush_work;
 	struct work_struct event_work;	/* used by dm to report failure event */
 	mempool_t *serial_info_pool;
 	void (*sync_super)(struct mddev *mddev, struct md_rdev *rdev);