[v2,06/19] btrfs: skip unnecessary delalloc search during fiemap and lseek

Commit Message

Filipe Manana Oct. 11, 2022, 12:16 p.m. UTC

From: Filipe Manana <[e-mail]>

During fiemap and lseek (hole and data seeking), there's no point in
iterating the inode's io tree to count delalloc bits if the inode's
delalloc bytes counter has a value of zero, as that counter is updated
whenever we set a range for delalloc or clear a range from delalloc.

So skip the counting and io tree iteration if the inode's delalloc bytes
counter has a value of zero. This helps save time when processing a file
range corresponding to a hole or prealloc (unwritten) extent.

This patch is part of a series comprised of the following patches:

  btrfs: get the next extent map during fiemap/lseek more efficiently
  btrfs: skip unnecessary extent map searches during fiemap and lseek
  btrfs: skip unnecessary delalloc search during fiemap and lseek

The following test was performed on a release kernel (Debian's default
kernel config) before and after applying those 3 patches.

   # Wrapper to call fiemap in extent count only mode.
   # (struct fiemap::fm_extent_count set to 0)
   $ cat fiemap.c
   #include <stdio.h>
   #include <unistd.h>
   #include <stdlib.h>
   #include <fcntl.h>
   #include <errno.h>
   #include <string.h>
   #include <sys/ioctl.h>
   #include <linux/fs.h>
   #include <linux/fiemap.h>

   int main(int argc, char **argv)
   {
            struct fiemap fiemap = { 0 };
            int fd;

            if (argc != 2) {
                    printf("usage: %s <path>\n", argv[0]);
                    return 1;
            }
            fd = open(argv[1], O_RDONLY);
            if (fd < 0) {
                    fprintf(stderr, "error opening file: %s\n",
                            strerror(errno));
                    return 1;
            }

            /* fiemap.fm_extent_count set to 0, to count extents only. */
            fiemap.fm_length = FIEMAP_MAX_OFFSET;
            if (ioctl(fd, FS_IOC_FIEMAP, &fiemap) < 0) {
                    fprintf(stderr, "fiemap error: %s\n",
                            strerror(errno));
                    return 1;
            }
            close(fd);
            printf("fm_mapped_extents = %d\n", fiemap.fm_mapped_extents);

            return 0;
   }

   $ gcc -o fiemap fiemap.c

And the wrapper shell script that creates a file with many holes and runs
fiemap against it:

   $ cat test.sh
   #!/bin/bash

   DEV=/dev/sdi
   MNT=/mnt/sdi

   mkfs.btrfs -f $DEV
   mount $DEV $MNT

   FILE_SIZE=$((1 * 1024 * 1024 * 1024))
   echo -n > $MNT/foobar
   for ((off = 0; off < $FILE_SIZE; off += 8192)); do
           xfs_io -c "pwrite -S 0xab $off 4K" $MNT/foobar > /dev/null
   done

   # flush all delalloc
   sync

   start=$(date +%s%N)
   ./fiemap $MNT/foobar
   end=$(date +%s%N)
   dur=$(( (end - start) / 1000000 ))
   echo "fiemap took $dur milliseconds"

   umount $MNT

Result before applying patchset:

   fm_mapped_extents = 131072
   fiemap took 63 milliseconds

Result after applying patchset:

   fm_mapped_extents = 131072
   fiemap took 39 milliseconds   (-38.1%)

Running the same test for a 512M file instead of a 1G file, gave the
following results.

Result before applying patchset:

   fm_mapped_extents = 65536
   fiemap took 29 milliseconds

Result after applying patchset:

   fm_mapped_extents = 65536
   fiemap took 20 milliseconds    (-31.0%)

Signed-off-by: Filipe Manana <[e-mail]>
---
 fs/btrfs/file.c | 33 ++++++++++++++++++++-------------
 1 file changed, 20 insertions(+), 13 deletions(-)

Patch

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 36618ddadc5f..0d0c662007ab 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3518,15 +3518,27 @@  static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end
 	struct extent_map *em;
 	u64 em_end;
 	u64 delalloc_len;
+	unsigned int outstanding_extents;
 
 	/*
 	 * Search the io tree first for EXTENT_DELALLOC. If we find any, it
 	 * means we have delalloc (dirty pages) for which writeback has not
 	 * started yet.
 	 */
-	*delalloc_start_ret = start;
-	delalloc_len = count_range_bits(&inode->io_tree, delalloc_start_ret, end,
-					len, EXTENT_DELALLOC, 1);
+	spin_lock(&inode->lock);
+	outstanding_extents = inode->outstanding_extents;
+
+	if (inode->delalloc_bytes > 0) {
+		spin_unlock(&inode->lock);
+		*delalloc_start_ret = start;
+		delalloc_len = count_range_bits(&inode->io_tree,
+						delalloc_start_ret, end,
+						len, EXTENT_DELALLOC, 1);
+	} else {
+		spin_unlock(&inode->lock);
+		delalloc_len = 0;
+	}
+
 	/*
 	 * If delalloc was found then *delalloc_start_ret has a sector size
 	 * aligned value (rounded down).
@@ -3534,17 +3546,12 @@  static bool find_delalloc_subrange(struct btrfs_inode *inode, u64 start, u64 end
 	if (delalloc_len > 0)
 		*delalloc_end_ret = *delalloc_start_ret + delalloc_len - 1;
 
-	spin_lock(&inode->lock);
-	if (inode->outstanding_extents == 0) {
-		/*
-		 * No outstanding extents means we don't have any delalloc that
-		 * is flushing, so return the unflushed range found in the io
-		 * tree (if any).
-		 */
-		spin_unlock(&inode->lock);
+	/*
+	 * No outstanding extents means we don't have any delalloc that is
+	 * flushing, so return the unflushed range found in the io tree (if any).
+	 */
+	if (outstanding_extents == 0)
 		return (delalloc_len > 0);
-	}
-	spin_unlock(&inode->lock);
 
 	/*
 	 * Now also check if there's any extent map in the range that does not