Btrfs: don't do nocow check unless we have to

Before we write into prealloc/nocow space we have to make sure that there are no
references to the extents we are writing into, which means checking the extent
tree and csum tree in the case of nocow.  So we don't want to do the nocow dance
unless we can't reserve data space, since it's a serious drag on performance.
With the following sequence

fallocate -l10737418240 /mnt/btrfs-test/file
cp --reflink /mnt/btrfs-test/file /mnt/btrfs-test/link
fio --name=randwrite --rw=randwrite --bs=4k --filename=/mnt/btrfs-test/file \
	--end_fsync=1

we get the worst case scenario where we have to fall back on to doing the check
anyway.

Without this patch
lat (usec): min=5, max=111598, avg=27.65, stdev=124.51
write: io=10240MB, bw=126876KB/s, iops=31718, runt= 82646msec

With this patch
lat (usec): min=3, max=91210, avg=14.09, stdev=110.62
write: io=10240MB, bw=212753KB/s, iops=53188, runt= 49286msec

We get twice the throughput, half of the runtime, and half of the average
latency.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
[ PAGE_CACHE_ removal related fixups ]
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>
(cherry picked from commit c6887cd11149d7325328749f06719071e6c725c6)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 700dccfff8b41bc5f6ee0da6e3cb8a42d11411a6..960b1094e6358cb30714567285e1362479a5e537 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1534,30 +1534,30 @@ static noinline ssize_t __btrfs_buffered_write(struct file *file,
                reserve_bytes = round_up(write_bytes + sector_offset,
                                root->sectorsize);
 
-               if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
-                                             BTRFS_INODE_PREALLOC)) &&
-                   check_can_nocow(inode, pos, &write_bytes) > 0) {
-                       /*
-                        * For nodata cow case, no need to reserve
-                        * data space.
-                        */
-                       only_release_metadata = true;
-                       /*
-                        * our prealloc extent may be smaller than
-                        * write_bytes, so scale down.
-                        */
-                       num_pages = DIV_ROUND_UP(write_bytes + offset,
-                                                PAGE_SIZE);
-                       reserve_bytes = round_up(write_bytes + sector_offset,
-                                       root->sectorsize);
-                       goto reserve_metadata;
-               }
-
                ret = btrfs_check_data_free_space(inode, pos, write_bytes);
-               if (ret < 0)
-                       break;
+               if (ret < 0) {
+                       if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
+                                                     BTRFS_INODE_PREALLOC)) &&
+                           check_can_nocow(inode, pos, &write_bytes) > 0) {
+                               /*
+                                * For nodata cow case, no need to reserve
+                                * data space.
+                                */
+                               only_release_metadata = true;
+                               /*
+                                * our prealloc extent may be smaller than
+                                * write_bytes, so scale down.
+                                */
+                               num_pages = DIV_ROUND_UP(write_bytes + offset,
+                                                        PAGE_SIZE);
+                               reserve_bytes = round_up(write_bytes +
+                                                        sector_offset,
+                                                        root->sectorsize);
+                       } else {
+                               break;
+                       }
+               }
 
-reserve_metadata:
                ret = btrfs_delalloc_reserve_metadata(inode, reserve_bytes);
                if (ret) {
                        if (!only_release_metadata)