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btrfs: reduce kmap_atomic time for checksumming 

Since commit c40a3d38af ("Btrfs: Compute and look up csums based on
sectorsized blocks") we do a kmap_atomic() on the contents of a bvec.
The code before c40a3d38af had the kmap region just around the
checksumming too.

kmap_atomic() in turn does a preempt_disable() and pagefault_disable(),
so we shouldn't map the data for too long. Reduce the time the bvec's
page is mapped to when we actually need it.

Performance wise it doesn't seem to make a huge difference with a 2 vcpu VM
on a /dev/zram device:

       vanilla      patched      delta
write  17.4MiB/s    17.8MiB/s	+0.4MiB/s (+2%)
read   40.6MiB/s    41.5MiB/s   +0.9MiB/s (+2%)

The following fio job profile was used in the comparision:

[global]
ioengine=libaio
direct=1
sync=1
norandommap
time_based
runtime=10m
size=100m
group_reporting
numjobs=2

[test]
filename=/mnt/test/fio
rw=randrw
rwmixread=70

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Johannes Thumshirn 2019-03-07 17:14:00 +01:00 committed by David Sterba
parent a1d198478e
commit 443c8e2a83
1 changed files with 2 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
fs/btrfs/file-item.c
View File

@ -458,8 +458,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
BUG_ON(!ordered); /* Logic error */
}
data = kmap_atomic(bvec.bv_page);
nr_sectors = BTRFS_BYTES_TO_BLKS(fs_info,
bvec.bv_len + fs_info->sectorsize
- 1);
@ -469,7 +467,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
offset < ordered->file_offset) {
unsigned long bytes_left;
kunmap_atomic(data);
sums->len = this_sum_bytes;
this_sum_bytes = 0;
btrfs_add_ordered_sum(inode, ordered, sums);
@ -489,16 +486,16 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
sums->bytenr = ((u64)bio->bi_iter.bi_sector << 9)
+ total_bytes;
index = 0;
data = kmap_atomic(bvec.bv_page);
}
sums->sums[index] = ~(u32)0;
data = kmap_atomic(bvec.bv_page);
sums->sums[index]
= btrfs_csum_data(data + bvec.bv_offset
+ (i * fs_info->sectorsize),
sums->sums[index],
fs_info->sectorsize);
kunmap_atomic(data);
btrfs_csum_final(sums->sums[index],
(char *)(sums->sums + index));
index++;
@ -507,7 +504,6 @@ blk_status_t btrfs_csum_one_bio(struct inode *inode, struct bio *bio,
total_bytes += fs_info->sectorsize;
}
kunmap_atomic(data);
}
this_sum_bytes = 0;
btrfs_add_ordered_sum(inode, ordered, sums);