btrfs: reduce kmap_atomic time for checksumming

Since commit c40a3d38aff4 ("Btrfs: Compute and look up csums based on
sectorsized blocks") we do a kmap_atomic() on the contents of a bvec.
The code before c40a3d38aff4 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>

diff --git a/fs/btrfs/file-item.c b/fs/btrfs/file-item.c
index cccc75d15970cbc61e8e1bcd1735fb28dc549123..6fccac9eab969393bb19a93d42e6a62ad3c72bd4 100644 (file)
--- a/fs/btrfs/file-item.c
+++ b/fs/btrfs/file-item.c
@@ -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);