From 443c8e2a839f18084cfdb5b0b62d90b8159f48ae Mon Sep 17 00:00:00 2001 From: Johannes Thumshirn Date: Thu, 7 Mar 2019 17:14:00 +0100 Subject: [PATCH] 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 Reviewed-by: David Sterba Signed-off-by: David Sterba --- 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 cccc75d15970..6fccac9eab96 100644 --- 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);