Re: [PATCH v2] f2fs: issue multi-device flushes in parallel

[Chao Yu]

On 8/6/26 11:08, Yonggil Song wrote:
> 
> On a multi-device setup, submit_flush_wait() walked the dirty devices
> in order and aborted the whole loop on the first device whose flush
> failed, leaving the remaining dirty devices un-flushed. Each device
> still needs its own data made durable, so a failure on one device must
> not skip the others. It also waited for one device's flush to complete
> before issuing the next, even though the devices have independent
> flush queues and could be flushed concurrently.
> 
> Flush every dirty device best-effort and in parallel instead: build
> one PREFLUSH bio per dirty device, submit them all, then wait for
> every completion, returning the first error seen (0 if all succeed).
> This bounds the flush window by the slowest device rather than the sum
> of all of them. No caller depends on the previous early-abort
> behaviour -- fsync only checks whether the return value is zero
> (fs/f2fs/file.c). The checkpoint path (f2fs_flush_device_cache) is
> unaffected; this only touches the fsync flush path.
> 
> The per-device bio/completion array is small and bounded (at most
> MAX_DEVICES entries), so allocate it with __GFP_NOFAIL rather than
> keeping a separate serial fallback path for allocation failure.
> 
> Signed-off-by: Yonggil Song <yonggil.song@xxxxxxxxxxx>
> ---
>  fs/f2fs/segment.c | 48 ++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 45 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index d71ddb3ee918..f3d3343e012f 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -566,21 +566,63 @@ static int __submit_flush_wait(struct f2fs_sb_info *sbi,
>  	return ret;
>  }
>  
> +static void f2fs_flush_end_io(struct bio *bio)
> +{
> +	complete(bio->bi_private);
> +}
> +
> +struct f2fs_flush_bio {
> +	struct bio bio;
> +	struct completion wait;
> +};
> +
>  static int submit_flush_wait(struct f2fs_sb_info *sbi, nid_t ino)
>  {
> +	struct f2fs_flush_bio *flush_bio;
> +	unsigned long devices = 0;
>  	int ret = 0;
>  	int i;
>  
>  	if (!f2fs_is_multi_device(sbi))
>  		return __submit_flush_wait(sbi, sbi->sb->s_bdev);
>  
> +	flush_bio = kmalloc(array_size(sbi->s_ndevs, sizeof(*flush_bio)),
> +				GFP_NOFS | __GFP_NOFAIL);
> +
>  	for (i = 0; i < sbi->s_ndevs; i++) {
>  		if (!f2fs_is_dirty_device(sbi, ino, i, FLUSH_INO))
>  			continue;
> -		ret = __submit_flush_wait(sbi, FDEV(i).bdev);
> -		if (ret)
> -			break;
> +
> +		bio_init(&flush_bio[i].bio, FDEV(i).bdev, NULL, 0,
> +			 REQ_OP_WRITE | REQ_SYNC | REQ_PREFLUSH);
> +		init_completion(&flush_bio[i].wait);
> +		flush_bio[i].bio.bi_private = &flush_bio[i].wait;
> +		flush_bio[i].bio.bi_end_io = f2fs_flush_end_io;
> +		devices |= BIT(i);

Can we submit bio here?

Thanks,

> +	}
> +
> +	for (i = 0; i < sbi->s_ndevs; i++) {
> +		if (devices & BIT(i))
> +			submit_bio(&flush_bio[i].bio);
> +	}
> +
> +	for (i = 0; i < sbi->s_ndevs; i++) {
> +		int err;
> +
> +		if (!(devices & BIT(i)))
> +			continue;
> +
> +		wait_for_completion(&flush_bio[i].wait);
> +		err = blk_status_to_errno(flush_bio[i].bio.bi_status);
> +		trace_f2fs_issue_flush(FDEV(i).bdev, test_opt(sbi, NOBARRIER),
> +				       test_opt(sbi, FLUSH_MERGE), err);
> +		if (!err)
> +			f2fs_update_iostat(sbi, NULL, FS_FLUSH_IO, 0);
> +		else if (!ret)
> +			ret = err;
> +		bio_uninit(&flush_bio[i].bio);
>  	}
> +	kfree(flush_bio);
>  	return ret;
>  }
>