[PATCH v2 2/7] dm-verity: make dm_verity_fec_io::bufs variable-length

Eric Biggers
2025-12-19T19:32:21Z

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@kernel.org>
---
drivers/md/dm-verity-fec.c | 44 +++++++++++++++++++-------------------
drivers/md/dm-verity-fec.h | 15 +++++++------
2 files changed, 31 insertions(+), 28 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index 2c1544556a1c..6d0b5b4b2699 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -8,10 +8,22 @@
#include "dm-verity-fec.h"
#include <linux/math64.h>

#define DM_MSG_PREFIX "verity-fec"

+/*
+ * When correcting a data block, the FEC code performs optimally when it can
+ * collect all the associated RS blocks at the same time. As each byte is part
+ * of a different RS block, there are '1 << data_dev_block_bits' RS blocks.
+ * There are '1 << DM_VERITY_FEC_BUF_RS_BITS' RS blocks per buffer, so that
+ * gives '1 << (data_dev_block_bits - DM_VERITY_FEC_BUF_RS_BITS)' buffers.
+ */
+static inline unsigned int fec_max_nbufs(struct dm_verity *v)
+{
+ return 1 << (v->data_dev_block_bits - DM_VERITY_FEC_BUF_RS_BITS);
+}
+
/*
* If error correction has been configured, returns true.
*/
bool verity_fec_is_enabled(struct dm_verity *v)
{
@@ -57,18 +69,10 @@ static u8 *fec_read_parity(struct dm_verity *v, u64 rsb, int index,
}

return res;
}

-/* Loop over each preallocated buffer slot. */
-#define fec_for_each_prealloc_buffer(__i) \
- for (__i = 0; __i < DM_VERITY_FEC_BUF_PREALLOC; __i++)
-
-/* Loop over each extra buffer slot. */
-#define fec_for_each_extra_buffer(io, __i) \
- for (__i = DM_VERITY_FEC_BUF_PREALLOC; __i < DM_VERITY_FEC_BUF_MAX; __i++)
-
/* Loop over each allocated buffer. */
#define fec_for_each_buffer(io, __i) \
for (__i = 0; __i < (io)->nbufs; __i++)

/* Loop over each RS block in each allocated buffer. */
@@ -305,24 +309,22 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
* Additional buffers are also allocated opportunistically to improve error
* correction performance, but these aren't required to succeed.
*/
static struct dm_verity_fec_io *fec_alloc_and_init_io(struct dm_verity *v)
{
+ const unsigned int max_nbufs = fec_max_nbufs(v);
struct dm_verity_fec *f = v->fec;
struct dm_verity_fec_io *fio;
unsigned int n;

fio = mempool_alloc(&f->fio_pool, GFP_NOIO);
fio->rs = mempool_alloc(&f->rs_pool, GFP_NOIO);

- memset(fio->bufs, 0, sizeof(fio->bufs));
-
- fec_for_each_prealloc_buffer(n)
- fio->bufs[n] = mempool_alloc(&f->prealloc_pool, GFP_NOIO);
+ fio->bufs[0] = mempool_alloc(&f->prealloc_pool, GFP_NOIO);

/* try to allocate the maximum number of buffers */
- fec_for_each_extra_buffer(fio, n) {
+ for (n = 1; n < max_nbufs; n++) {
fio->bufs[n] = kmem_cache_alloc(f->cache, GFP_NOWAIT);
/* we can manage with even one buffer if necessary */
if (unlikely(!fio->bufs[n]))
break;
}
@@ -460,16 +462,14 @@ void __verity_fec_finish_io(struct dm_verity_io *io)
struct dm_verity_fec *f = io->v->fec;
struct dm_verity_fec_io *fio = io->fec_io;

mempool_free(fio->rs, &f->rs_pool);

- fec_for_each_prealloc_buffer(n)
- mempool_free(fio->bufs[n], &f->prealloc_pool);
+ mempool_free(fio->bufs[0], &f->prealloc_pool);

- fec_for_each_extra_buffer(fio, n)
- if (fio->bufs[n])
- kmem_cache_free(f->cache, fio->bufs[n]);
+ for (n = 1; n < fio->nbufs; n++)
+ kmem_cache_free(f->cache, fio->bufs[n]);

mempool_free(fio->output, &f->output_pool);

mempool_free(fio, &f->fio_pool);
io->fec_io = NULL;
@@ -733,11 +733,12 @@ int verity_fec_ctr(struct dm_verity *v)
return -E2BIG;
}

/* Preallocate some dm_verity_fec_io structures */
ret = mempool_init_kmalloc_pool(&f->fio_pool, num_online_cpus(),
- sizeof(struct dm_verity_fec_io));
+ struct_size((struct dm_verity_fec_io *)0,
+ bufs, fec_max_nbufs(v)));
if (ret) {
ti->error = "Cannot allocate FEC IO pool";
return ret;
}

@@ -755,13 +756,12 @@ int verity_fec_ctr(struct dm_verity *v)
if (!f->cache) {
ti->error = "Cannot create FEC buffer cache";
return -ENOMEM;
}

- /* Preallocate DM_VERITY_FEC_BUF_PREALLOC buffers for each thread */
- ret = mempool_init_slab_pool(&f->prealloc_pool, num_online_cpus() *
- DM_VERITY_FEC_BUF_PREALLOC,
+ /* Preallocate one buffer for each thread */
+ ret = mempool_init_slab_pool(&f->prealloc_pool, num_online_cpus(),
f->cache);
if (ret) {
ti->error = "Cannot allocate FEC buffer prealloc pool";
return ret;
}
diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index b9488d1ddf14..571097438311 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -15,15 +15,11 @@
#define DM_VERITY_FEC_RSM 255
#define DM_VERITY_FEC_MAX_RSN 253
#define DM_VERITY_FEC_MIN_RSN 231 /* ~10% space overhead */

/* buffers for deinterleaving and decoding */
-#define DM_VERITY_FEC_BUF_PREALLOC 1 /* buffers to preallocate */
#define DM_VERITY_FEC_BUF_RS_BITS 4 /* 1 << RS blocks per buffer */
-/* we need buffers for at most 1 << block size RS blocks */
-#define DM_VERITY_FEC_BUF_MAX \
- (1 << (PAGE_SHIFT - DM_VERITY_FEC_BUF_RS_BITS))

#define DM_VERITY_OPT_FEC_DEV "use_fec_from_device"
#define DM_VERITY_OPT_FEC_BLOCKS "fec_blocks"
#define DM_VERITY_OPT_FEC_START "fec_start"
#define DM_VERITY_OPT_FEC_ROOTS "fec_roots"
@@ -50,14 +46,21 @@ struct dm_verity_fec {

/* per-bio data */
struct dm_verity_fec_io {
struct rs_control *rs; /* Reed-Solomon state */
int erasures[DM_VERITY_FEC_MAX_RSN]; /* erasures for decode_rs8 */
- u8 *bufs[DM_VERITY_FEC_BUF_MAX]; /* bufs for deinterleaving */
- unsigned int nbufs; /* number of buffers allocated */
u8 *output; /* buffer for corrected output */
unsigned int level; /* recursion level */
+ unsigned int nbufs; /* number of buffers allocated */
+ /*
+ * Buffers for deinterleaving RS blocks. Each buffer has space for
+ * the data bytes of (1 << DM_VERITY_FEC_BUF_RS_BITS) RS blocks. The
+ * array length is fec_max_nbufs(v), and we try to allocate that many
+ * buffers. However, in low-memory situations we may be unable to
+ * allocate all buffers. 'nbufs' holds the number actually allocated.
+ */
+ u8 *bufs[];
};

#ifdef CONFIG_DM_VERITY_FEC

/* each feature parameter requires a value */
--
2.52.0