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Date: Mon, 6 Oct 2014 20:59:04 -0700
From: "Darrick J. Wong" <darrick.wong@...cle.com>
To: Xiaoguang Wang <wangxg.fnst@...fujitsu.com>
Cc: linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH] filefrag: fix wrong extent count calculation when using
FIBMAP
On Tue, Oct 07, 2014 at 11:50:34AM +0800, Xiaoguang Wang wrote:
> Hi,
>
> On 10/07/2014 10:43 AM, Darrick J. Wong wrote:
> > On Mon, Oct 06, 2014 at 07:05:39PM +0800, Xiaoguang Wang wrote:
> >> When using FIBMAP and '-e' option is specified, the calculation for fiemap_extent
> >> is wrong, we wrongly updated fm_ext.fe_logical for every iteration, please see the
> >> code in the end of 'for' loop in fm_ext.fe_logical().
> >>
> >> In an ext2 file system(block size is 1024 bytes),
> >> dd if=/dev/zero of=testfile bs=1k count=15
> >> Using debugfs, corresponding physical blocks are "2049 2050 2051 2052 2053 2054
> >> 2055 2056 2057 2058 2059 2060 1025 2061 2062 2063", 1025 is this indirect block.
> >> Before this patch, filefrag's output would be:
> >> filefrag -B -e testfile
> >> Filesystem type is: ef53
> >> Filesystem cylinder groups approximately 16
> >> File size of testfile is 15360 (15 blocks of 1024 bytes)
> >> ext: logical_offset: physical_offset: length: expected: flags:
> >> 0: 1.. 2: 2050.. 2051: 2: 2051: merged
> >> 1: 3.. 4: 2052.. 2053: 2: 2053: merged
> >> 2: 5.. 6: 2054.. 2055: 2: 2055: merged
> >> 3: 7.. 8: 2056.. 2057: 2: 2057: merged
> >> 4: 9.. 10: 2058.. 2059: 2: 2059: merged
> >> 5: 11.. 12: 2060.. 2061: 2: 2062: merged
> >> 6: 13.. 14: 2062.. 2063: 2: 2063: merged,eof
> >> 7: 14.. 14: 2063.. 2063: 1: 2063: merged,eof
> >> This output is not reasonable.
> >
> > It's just plain whacky. Why would logical offset 14 be listed twice? :)
>
> Because of the wrong code :) It updates fm_ext.fe_logical and fm_ext.fe_physical for every
> iteration :) and i think the original code is not that readable.
> >
> >> Fix this bug and try to make it readable. After this patch, the output would be:
> >> ./filefrag -B -e mntpoint/testfile
> >> Filesystem type is: ef53
> >> Filesystem cylinder groups approximately 16
> >> File size of mntpoint/testfile is 15360 (15 blocks of 1024 bytes)
> >> ext: logical_offset: physical_offset: length: expected: flags:
> >> 0: 0.. 11: 2049.. 2060: 12: 2062: merged
> >> 1: 12.. 14: 2061.. 2063: 3: 2063: merged,eof
> >> mntpoint/testfile: 2 extents found, perfection would be 1 extent
> >
> > Where'd the indirect block end up, if not in the middle of 2049-2063?
>
> Indirect block's information is used to judge whether logical region is continuous physically.
> For example, the above testfile's first indirect block is 1025, and will not shown in the output.
1025? Huh. Is this an old FS, or is there something wrong with the block
allocator?
--D
>
> >
> >> Signed-off-by: Xiaoguang Wang <wangxg.fnst@...fujitsu.com>
> >> ---
> >> misc/filefrag.c | 37 +++++++++++++++++++++++--------------
> >> 1 file changed, 23 insertions(+), 14 deletions(-)
> >>
> >> diff --git a/misc/filefrag.c b/misc/filefrag.c
> >> index c1a8684..e9f7e68 100644
> >> --- a/misc/filefrag.c
> >> +++ b/misc/filefrag.c
> >> @@ -315,32 +315,41 @@ static int filefrag_fibmap(int fd, int blk_shift, int *num_extents,
> >> return rc;
> >> if (block == 0)
> >> continue;
> >> + count++;
> >> +
> >> if (*num_extents == 0) {
> >> (*num_extents)++;
> >> if (force_extent) {
> >> print_extent_header();
> >> + fm_ext.fe_logical = logical;
> >> fm_ext.fe_physical = block * st->st_blksize;
> >> + fm_ext.fe_length = st->st_blksize;
> >> }
> >> + last_block = block;
> >> + continue;
> >> }
> >> - count++;
> >> - if (force_extent && last_block != 0 &&
> >> - (block != last_block + 1 ||
> >> - fm_ext.fe_logical + fm_ext.fe_length != logical)) {
> >> - print_extent_info(&fm_ext, *num_extents - 1,
> >> - (last_block + 1) * st->st_blksize,
> >> - blk_shift, st);
> >> - fm_ext.fe_length = 0;
> >> - (*num_extents)++;
> >> +
> >> + if (force_extent) {
> >> + if (block != last_block + 1 ||
> >> + fm_ext.fe_length + fm_ext.fe_logical != logical) {
> >> + print_extent_info(&fm_ext, *num_extents - 1,
> >> + (last_block + 1) *
> >> + st->st_blksize,
> >> + blk_shift, st);
> >> + fm_ext.fe_logical = logical;
> >> + fm_ext.fe_physical = block * st->st_blksize;
> >> + fm_ext.fe_length = st->st_blksize;
> >> + (*num_extents)++;
> >> + } else {
> >> + fm_ext.fe_length += st->st_blksize;
> >> + }
> >> } else if (last_block && (block != last_block + 1)) {
> >> - if (verbose)
> >> + if (verbose) {
> >> printf("Discontinuity: Block %ld is at %lu (was "
> >> "%lu)\n", i, block, last_block + 1);
> >> - fm_ext.fe_length = 0;
> >> + }
> >
> > The { } is not needed for a single line if statement.
>
> OK, I'll send a new version to remove it, thanks!
>
> Regards,
> Xiaoguang Wang
> >
> >> (*num_extents)++;
> >> }
> >> - fm_ext.fe_logical = logical;
> >> - fm_ext.fe_physical = block * st->st_blksize;
> >> - fm_ext.fe_length += st->st_blksize;
> >> last_block = block;
> >
> > Otherwise, I think this looks decent.
> >
> > --D
> >
> >> }
> >>
> >> --
> >> 1.8.2.1
> >>
> >> --
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> > .
> >
>
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