linux-kernel - Re: [PATCH v2 2/7] dm-verity: make dm_verity_fec

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Message-ID: <20251219200708.GA2662043@google.com>
Date: Fri, 19 Dec 2025 20:07:08 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Eric Biggers <ebiggers@...nel.org>
Cc: 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>,
	Eran Messeri <eranm@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] dm-verity: make dm_verity_fec_io::bufs
 variable-length

On Fri, Dec 19, 2025 at 11:29:04AM -0800, Eric Biggers wrote:
> When correcting a data block, the FEC code performs optimally when it
> has enough buffers to hold all the needed RS blocks.  That number of
> buffers is '1 << (v->data_dev_block_bits - DM_VERITY_FEC_BUF_RS_BITS)'.
> 
> However, since v->data_dev_block_bits isn't a compile-time constant, the
> code actually used PAGE_SHIFT instead.
> 
> With the traditional PAGE_SIZE == data_block_size == 4096, this was
> fine.  However, when PAGE_SIZE > data_block_size, this wastes space.
> E.g., with data_block_size == 4096 && PAGE_SIZE == 16384, struct
> dm_verity_fec_io is 9240 bytes, when in fact only 3096 bytes are needed.
> 
> Fix this by making dm_verity_fec_io::bufs a variable-length array.
> 
> This makes the macros DM_VERITY_FEC_BUF_MAX and
> fec_for_each_extra_buffer() no longer apply, so remove them.  For
> consistency, and because DM_VERITY_FEC_BUF_PREALLOC is fixed at 1 and
> was already assumed to be 1 (considering that mempool_alloc() shouldn't
> be called in a loop), also remove the related macros
> DM_VERITY_FEC_BUF_PREALLOC and fec_for_each_prealloc_buffer().
> 
> Signed-off-by: Eric Biggers <ebiggers@...nel.org>

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami