linux-kernel - [PATCH 13/22] dm-verity-fec: simplify computation of rsb

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Message-ID: <20260206045942.52965-14-ebiggers@kernel.org>
Date: Thu,  5 Feb 2026 20:59:32 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: dm-devel@...ts.linux.dev,
	Alasdair Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...nel.org>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Benjamin Marzinski <bmarzins@...hat.com>
Cc: Sami Tolvanen <samitolvanen@...gle.com>,
	linux-kernel@...r.kernel.org,
	Eric Biggers <ebiggers@...nel.org>
Subject: [PATCH 13/22] dm-verity-fec: simplify computation of rsb

To compute 'rsb', verity_fec_decode() divides 'offset' by
'v->fec->region_blocks << v->data_dev_block_bits', then subtracts the
quotient times that divisor.  That's simply the long way to do a modulo
operation, i.e. a - b * floor(a / b) instead of just a % b.  Use
div64_u64_rem() to get the remainder more concisely.

Signed-off-by: Eric Biggers <ebiggers@...nel.org>
---
 drivers/md/dm-verity-fec.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index 28b47497c3d3f..63eeef26a3999 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -375,11 +375,11 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
 		      enum verity_block_type type, const u8 *want_digest,
 		      sector_t block, u8 *dest)
 {
 	int r;
 	struct dm_verity_fec_io *fio;
-	u64 offset, res, rsb;
+	u64 offset, rsb;
 
 	if (!verity_fec_is_enabled(v))
 		return -EOPNOTSUPP;
 
 	fio = io->fec_io;
@@ -403,17 +403,17 @@ int verity_fec_decode(struct dm_verity *v, struct dm_verity_io *io,
 	 * and each code is interleaved over k blocks to make it less likely
 	 * that bursty corruption will leave us in unrecoverable state.
 	 */
 
 	offset = block << v->data_dev_block_bits;
-	res = div64_u64(offset, v->fec->region_blocks << v->data_dev_block_bits);
 
 	/*
 	 * The base RS block we can feed to the interleaver to find out all
 	 * blocks required for decoding.
 	 */
-	rsb = offset - res * (v->fec->region_blocks << v->data_dev_block_bits);
+	div64_u64_rem(offset, v->fec->region_blocks << v->data_dev_block_bits,
+		      &rsb);
 
 	/*
 	 * Locating erasures is slow, so attempt to recover the block without
 	 * them first. Do a second attempt with erasures if the corruption is
 	 * bad enough.
-- 
2.52.0