Re: [PATCH 3/3] btrfs: disguise single-data-RAID56 as RAID1/RAID1C3

[David Sterba <dsterba@suse.cz>]

On Sun, May 24, 2026 at 01:43:54PM +0930, Qu Wenruo wrote:
> I tried the idea, and it doesn't look elegant at all, we need a lot of 
> if () checks around every xor_gen()/raid6_call.
> 
> Sure the end result is not that huge (and I have only compile tested 
> it), but if we had some parameters passed wrongly, or some corner case 
> missed, it can be very hard to notice.
> 
> Just paste the adhoc fix for reference:

I ended up with something similar, but replaced the open coded ifs with
a helper so there's not that much churn at the call sites.

> diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> index 08ee8f316d96..50fefe33bbe3 100644
> --- a/fs/btrfs/raid56.c
> +++ b/fs/btrfs/raid56.c
> @@ -1410,11 +1410,17 @@ static void generate_pq_vertical_step(struct 
> btrfs_raid_bio *rbio, unsigned int
>   				rbio_qstripe_paddr(rbio, sector_nr, step_nr));
> 
>   		assert_rbio(rbio);
> -		raid6_call.gen_syndrome(rbio->real_stripes, step, pointers);
> +		if (rbio->nr_data > 1) {
> +			raid6_call.gen_syndrome(rbio->real_stripes, step, pointers);
> +		} else {
> +			memcpy(pointers[1], pointers[0], step);
> +			memcpy(pointers[2], pointers[0], step);
> +		}
>   	} else {
>   		/* raid5 */
>   		memcpy(pointers[rbio->nr_data], pointers[0], step);
> -		xor_gen(pointers[rbio->nr_data], pointers + 1, rbio->nr_data - 1,
> +		if (rbio->nr_data > 1)
> +			xor_gen(pointers[rbio->nr_data], pointers + 1, rbio->nr_data - 1,
>   				step);
>   	}
>   	for (stripe = stripe - 1; stripe >= 0; stripe--)
> @@ -1987,11 +1993,17 @@ static void recover_vertical_step(struct 
> btrfs_raid_bio *rbio,
>   		}
> 
>   		if (failb == rbio->real_stripes - 2) {
> -			raid6_datap_recov(rbio->real_stripes, step,
> -					  faila, pointers);
> +			if (unlikely(rbio->nr_data == 1))
> +				memcpy(pointers[0], pointers[rbio->real_stripes - 1], step);
> +			else
> +				raid6_datap_recov(rbio->real_stripes, step,
> +						  faila, pointers);
>   		} else {
> -			raid6_2data_recov(rbio->real_stripes, step,
> -					  faila, failb, pointers);
> +			if (unlikely(rbio->nr_data == 1))
> +				memcpy(pointers[0], pointers[rbio->real_stripes - 2], step);

The 2data recovery needs to do 2 memcpy

> +			else
> +				raid6_2data_recov(rbio->real_stripes, step,
> +						  faila, failb, pointers);
>   		}
>   	} else {
>   		void *p;
> @@ -2002,6 +2014,9 @@ static void recover_vertical_step(struct 
> btrfs_raid_bio *rbio,
>   		/* Copy parity block into failed block to start with */
>   		memcpy(pointers[faila], pointers[rbio->nr_data], step);
> 
> +		if (rbio->nr_data == 1)
> +			goto cleanup;
> +
>   		/* Rearrange the pointer array */
>   		p = pointers[faila];
>   		for (stripe_nr = faila; stripe_nr < rbio->nr_data - 1;
> @@ -2644,11 +2659,17 @@ static bool verify_one_parity_step(struct 
> btrfs_raid_bio *rbio,
>   	if (has_qstripe) {
>   		assert_rbio(rbio);
>   		/* RAID6, call the library function to fill in our P/Q. */
> -		raid6_call.gen_syndrome(rbio->real_stripes, step, pointers);
> +		if (rbio->nr_data > 1) {
> +			raid6_call.gen_syndrome(rbio->real_stripes, step, pointers);
> +		} else {
> +			memcpy(pointers[1], pointers[0], step);
> +			memcpy(pointers[2], pointers[0], step);
> +		}
>   	} else {
>   		/* RAID5. */
>   		memcpy(pointers[nr_data], pointers[0], step);
> -		xor_gen(pointers[nr_data], pointers + 1, nr_data - 1, step);
> +		if (rbio->nr_data > 1)
> +			xor_gen(pointers[nr_data], pointers + 1, nr_data - 1, step);
>   	}
> 
>   	/* Check scrubbing parity and repair it. */
> --
> 2.54.0