Since commit c40a3d38aff4 ("Btrfs: Compute and look up csums based on
sectorsized blocks") we do a kmap_atomic() on the contents of a bvec.
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@xxxxxxx>
---
fs/btrfs/file-item.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index 920bf3b4b0ef..aa40e66df1e1 100644
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -453,8 +453,6 @@ 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);
@@ -464,7 +462,6 @@ 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);
@@ -482,16 +479,16 @@ 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++;
@@ -500,7 +497,6 @@ 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);
--
2.16.4
