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Subject[PATCH v2] dm-integrity: Prevent RMW for full metadata buffer writes
If a full metadata buffer is being written, don't read it first. This
prevents a read-modify-write cycle and increases performance on HDDs
considerably.

To do this we now calculate the checksums for all sectors in the bio in one
go in integrity_metadata and then pass the result to dm_integrity_rw_tag,
which now checks if we overwrite the whole buffer.

Benchmarking with a 5400RPM HDD with bitmap mode:
integritysetup format --no-wipe --batch-mode --interleave-sectors $((64*1024)) -t 4 -s 512 -I crc32c -B /dev/sdc
integritysetup open --buffer-sectors=1 -I crc32c -B /dev/sdc hdda_integ
dd if=/dev/zero of=/dev/mapper/hdda_integ bs=64K count=$((16*1024*4)) conv=fsync oflag=direct status=progress

Without patch:
4294967296 bytes (4.3 GB, 4.0 GiB) copied, 400.326 s, 10.7 MB/s

With patch:
4294967296 bytes (4.3 GB, 4.0 GiB) copied, 41.2057 s, 104 MB/s

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
---
Hello Everyone,
So here is v2, now checking if we overwrite a whole metadata buffer instead
of the (buggy) check if we overwrite a whole tag area before.
Performance stayed the same (with --buffer-sectors=1).

The only quirk now is that it advertises a very big optimal io size in the
standard configuration (where buffer_sectors=128). Is this a Problem?

v2:
-fix dm_integrity_rw_tag to check if we overwrite a whole metadat buffer
-fix optimal io size to check if we overwrite a whole metadata buffer

Regards,
Lukas Straub

drivers/md/dm-integrity.c | 81 +++++++++++++++++++++++----------------
1 file changed, 47 insertions(+), 34 deletions(-)

diff --git a/drivers/md/dm-integrity.c b/drivers/md/dm-integrity.c
index b225b3e445fa..a6d3cf1406df 100644
--- a/drivers/md/dm-integrity.c
+++ b/drivers/md/dm-integrity.c
@@ -1309,9 +1309,17 @@ static int dm_integrity_rw_tag(struct dm_integrity_c *ic, unsigned char *tag, se
if (unlikely(r))
return r;

- data = dm_bufio_read(ic->bufio, *metadata_block, &b);
- if (IS_ERR(data))
- return PTR_ERR(data);
+ /* Don't read metadata sectors from disk if we're going to overwrite them completely */
+ if (op == TAG_WRITE && *metadata_offset == 0 \
+ && total_size >= (1U << SECTOR_SHIFT << ic->log2_buffer_sectors)) {
+ data = dm_bufio_new(ic->bufio, *metadata_block, &b);
+ if (IS_ERR(data))
+ return PTR_ERR(data);
+ } else {
+ data = dm_bufio_read(ic->bufio, *metadata_block, &b);
+ if (IS_ERR(data))
+ return PTR_ERR(data);
+ }

to_copy = min((1U << SECTOR_SHIFT << ic->log2_buffer_sectors) - *metadata_offset, total_size);
dp = data + *metadata_offset;
@@ -1514,6 +1522,8 @@ static void integrity_metadata(struct work_struct *w)
{
struct dm_integrity_io *dio = container_of(w, struct dm_integrity_io, work);
struct dm_integrity_c *ic = dio->ic;
+ unsigned sectors_to_process = dio->range.n_sectors;
+ sector_t sector = dio->range.logical_sector;

int r;

@@ -1522,16 +1532,14 @@ static void integrity_metadata(struct work_struct *w)
struct bio_vec bv;
unsigned digest_size = crypto_shash_digestsize(ic->internal_hash);
struct bio *bio = dm_bio_from_per_bio_data(dio, sizeof(struct dm_integrity_io));
- char *checksums;
+ char *checksums, *checksums_ptr;
unsigned extra_space = unlikely(digest_size > ic->tag_size) ? digest_size - ic->tag_size : 0;
char checksums_onstack[HASH_MAX_DIGESTSIZE];
- unsigned sectors_to_process = dio->range.n_sectors;
- sector_t sector = dio->range.logical_sector;

if (unlikely(ic->mode == 'R'))
goto skip_io;

- checksums = kmalloc((PAGE_SIZE >> SECTOR_SHIFT >> ic->sb->log2_sectors_per_block) * ic->tag_size + extra_space,
+ checksums = kmalloc((dio->range.n_sectors >> ic->sb->log2_sectors_per_block) * ic->tag_size + extra_space,
GFP_NOIO | __GFP_NORETRY | __GFP_NOWARN);
if (!checksums) {
checksums = checksums_onstack;
@@ -1542,49 +1550,45 @@ static void integrity_metadata(struct work_struct *w)
}
}

+ checksums_ptr = checksums;
__bio_for_each_segment(bv, bio, iter, dio->orig_bi_iter) {
unsigned pos;
- char *mem, *checksums_ptr;
-
-again:
+ char *mem;
mem = (char *)kmap_atomic(bv.bv_page) + bv.bv_offset;
pos = 0;
- checksums_ptr = checksums;
do {
integrity_sector_checksum(ic, sector, mem + pos, checksums_ptr);
- checksums_ptr += ic->tag_size;
- sectors_to_process -= ic->sectors_per_block;
+
+ if (likely(checksums != checksums_onstack)) {
+ checksums_ptr += ic->tag_size;
+ } else {
+ r = dm_integrity_rw_tag(ic, checksums, &dio->metadata_block, &dio->metadata_offset,
+ ic->tag_size, !dio->write ? TAG_CMP : TAG_WRITE);
+ if (unlikely(r))
+ goto internal_hash_error;
+ }
+
pos += ic->sectors_per_block << SECTOR_SHIFT;
sector += ic->sectors_per_block;
- } while (pos < bv.bv_len && sectors_to_process && checksums != checksums_onstack);
+ sectors_to_process -= ic->sectors_per_block;
+ } while (pos < bv.bv_len && sectors_to_process);
kunmap_atomic(mem);

- r = dm_integrity_rw_tag(ic, checksums, &dio->metadata_block, &dio->metadata_offset,
- checksums_ptr - checksums, !dio->write ? TAG_CMP : TAG_WRITE);
- if (unlikely(r)) {
- if (r > 0) {
- DMERR_LIMIT("Checksum failed at sector 0x%llx",
- (unsigned long long)(sector - ((r + ic->tag_size - 1) / ic->tag_size)));
- r = -EILSEQ;
- atomic64_inc(&ic->number_of_mismatches);
- }
- if (likely(checksums != checksums_onstack))
- kfree(checksums);
- goto error;
- }
-
if (!sectors_to_process)
break;
+ }

- if (unlikely(pos < bv.bv_len)) {
- bv.bv_offset += pos;
- bv.bv_len -= pos;
- goto again;
+ if (likely(checksums != checksums_onstack)) {
+ r = dm_integrity_rw_tag(ic, checksums, &dio->metadata_block, &dio->metadata_offset,
+ (dio->range.n_sectors >> ic->sb->log2_sectors_per_block) * ic->tag_size,
+ !dio->write ? TAG_CMP : TAG_WRITE);
+ if (unlikely(r)) {
+ kfree(checksums);
+ goto internal_hash_error;
}
+ kfree(checksums);
}

- if (likely(checksums != checksums_onstack))
- kfree(checksums);
} else {
struct bio_integrity_payload *bip = dio->orig_bi_integrity;

@@ -1615,6 +1619,13 @@ static void integrity_metadata(struct work_struct *w)
skip_io:
dec_in_flight(dio);
return;
+internal_hash_error:
+ if (r > 0) {
+ DMERR_LIMIT("Checksum failed at sector 0x%llx",
+ (unsigned long long)(sector - ((r + ic->tag_size - 1) / ic->tag_size)));
+ r = -EILSEQ;
+ atomic64_inc(&ic->number_of_mismatches);
+ }
error:
dio->bi_status = errno_to_blk_status(r);
dec_in_flight(dio);
@@ -3019,6 +3030,8 @@ static void dm_integrity_io_hints(struct dm_target *ti, struct queue_limits *lim
limits->physical_block_size = ic->sectors_per_block << SECTOR_SHIFT;
blk_limits_io_min(limits, ic->sectors_per_block << SECTOR_SHIFT);
}
+
+ blk_limits_io_opt(limits, 1U << SECTOR_SHIFT << ic->log2_buffer_sectors >> ic->log2_tag_size << SECTOR_SHIFT );
}

static void calculate_journal_section_size(struct dm_integrity_c *ic)
--
2.20.1
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