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SubjectRe: [dm-devel] [PATCH v2] dm-integrity: Prevent RMW for full metadata buffer writes
On Mon, 30 Mar 2020 12:34:04 -0400 (EDT)
Mikulas Patocka <mpatocka@redhat.com> wrote:

> Hi
>
> I tested it on my rotational disk:
>
>
> On Thu, 27 Feb 2020, Lukas Straub wrote:
>
> > 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
>
> I get 42.3 MB/s. (it seems that my disk has better prefetch than yours).
>
> > With patch:
> > 4294967296 bytes (4.3 GB, 4.0 GiB) copied, 41.2057 s, 104 MB/s
>
> I get 110 MB/s.
>
> > Signed-off-by: Lukas Straub <lukasstraub2@web.de>
>
> BUT: when I removed "--buffer-sectors=1" from the command line, I've got
> these numbers:
> without the patch: 101 MB/s

I get 86.5 MB/s.

So in my case it's worth it and as you saw it doesn't (negatively) affect other cases.

Regards,
Lukas Straub

> with the patch: 101 MB/s
>
> So, you crippled performance with "--buffer-sectors=1" and then made a
> patch to restore it.
>
> The patch only helps 10%.
>
> Mikulas
>
>
> > ---
> > 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
> >
> >
> > --
> > dm-devel mailing list
> > dm-devel@redhat.com
> > https://www.redhat.com/mailman/listinfo/dm-devel
> >
>

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