Re: [dm-devel] [PATCH v2] dm-integrity: Prevent RMW for full metadata buffer writes

[Lukas Straub <lukasstraub2@web.de>]

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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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