On Sat, 24 Nov 2012, Theodore Ts'o wrote:
> Date: Sat, 24 Nov 2012 19:36:29 -0500
> From: Theodore Ts'o <tytso@xxxxxxx>
> To: Ext4 Developers List <linux-ext4@xxxxxxxxxxxxxxx>
> Cc: Theodore Ts'o <tytso@xxxxxxx>
> Subject: [PATCH 1/6] libext2fs: optimize rb_set_bmap_range()
>
> This speeds up reading bitmaps from disk for very large (and full)
> disks by significant amounts (i.e., up to two CPU minutes for a 4T
> file system).
>
> Addresses-Google-Bug: #7534813
>
> Signed-off-by: "Theodore Ts'o" <tytso@xxxxxxx>
> ---
> lib/ext2fs/blkmap64_rb.c | 32 +++++++++++++++++++++++++++++---
> 1 file changed, 29 insertions(+), 3 deletions(-)
>
> diff --git a/lib/ext2fs/blkmap64_rb.c b/lib/ext2fs/blkmap64_rb.c
> index a42eda1..816f44f 100644
> --- a/lib/ext2fs/blkmap64_rb.c
> +++ b/lib/ext2fs/blkmap64_rb.c
> @@ -674,16 +674,42 @@ static errcode_t rb_set_bmap_range(ext2fs_generic_bitmap bitmap,
> __u64 start, size_t num, void *in)
> {
> struct ext2fs_rb_private *bp;
> + unsigned char *cp;
> size_t i;
> + int first_set = -1;
> int ret;
>
> bp = (struct ext2fs_rb_private *) bitmap->private;
>
> for (i = 0; i < num; i++) {
> - ret = ext2fs_test_bit(i, in);
> - if (ret)
> - rb_insert_extent(start + i - bitmap->start, 1, bp);
> + if (i & 7 == 0) {
> + unsigned char c = cp[i/8];
I do not see cp initialized anywhere. I suppose it should map to the
'in' bitmap ?
I guess that 8 will always be aliquot part of 'num', by maybe we
could explicitly check for that to avoid access to uninitialized
memory ?
> + if (c == 0xFF) {
> + if (first_set == -1)
> + first_set = i;
> + i += 7;
> + continue;
> + }
> + if ((c == 0x00) && (first_set == -1)) {
> + i += 7;
> + continue;
> + }
> + }
> + if (ext2fs_test_bit(i, in)) {
> + if (first_set == -1)
> + first_set = i;
> + continue;
> + }
> + if (first_set == -1)
> + continue;
> +
> + rb_insert_extent(start + first_set - bitmap->start,
> + i - first_set, bp);
> + first_set = -1;
> }
> + if (first_set != -1)
> + rb_insert_extent(start + first_set - bitmap->start,
> + num - first_set, bp);
>
> return 0;
> }
>
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