[dm-devel] [PATCH] dm-integrity: Prevent RMW for full tag area writes
Mikulas Patocka
mpatocka at redhat.com
Tue Feb 25 16:41:45 UTC 2020
On Thu, 20 Feb 2020, Lukas Straub wrote:
> If a full tag area 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 tag area.
>
> Benchmarking with a 5400RPM HDD with bitmap mode:
> integritysetup format --no-wipe --batch-mode --interleave-sectors $((64*1024)) -t 4 -s 512 -I crc32c -B /dev/sdc
> integritysetup open -I crc32c -B /dev/sdc hdda_integ
> dd if=/dev/zero of=/dev/mapper/hdda_integ bs=64K count=$((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 at web.de>
> ---
> drivers/md/dm-integrity.c | 80 ++++++++++++++++++++++-----------------
> 1 file changed, 46 insertions(+), 34 deletions(-)
>
> diff --git a/drivers/md/dm-integrity.c b/drivers/md/dm-integrity.c
> index b225b3e445fa..0e5ddcf44935 100644
> --- a/drivers/md/dm-integrity.c
> +++ b/drivers/md/dm-integrity.c
> @@ -1309,9 +1309,16 @@ 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 tag area from disk if we're going to overwrite it completely */
> + if (op == TAG_WRITE && *metadata_offset == 0 && total_size >= ic->metadata_run) {
Hi
This is incorrect logic because ic->metadata_run is in the units of
512-byte sectors and total_size is in bytes.
If I correct the bug and change it to "if (op == TAG_WRITE &&
*metadata_offset == 0 && total_size >= ic->metadata_run << SECTOR_SHIFT)",
then the benchmark doesn't show any performance advantage at all.
You would need much bigger bios to take advantage for this - for example,
if we have 4k block size and 64k metadata buffer size and 4-byte crc32,
there are 65536/4=16384 tags in one metadata buffer and we would need
16384*4096=64MiB bio to completely overwrite the metadata buffer. Such big
bios are not realistic.
Mikulas
> + 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 +1521,8 @@ 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 +1531,14 @@ 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 +1549,45 @@ 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 +1618,13 @@ 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 +3029,8 @@ 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 << ic->sb->log2_interleave_sectors));
> }
>
> static void calculate_journal_section_size(struct dm_integrity_c *ic)
> --
> 2.20.1
>
>
> --
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