linux-ext4 - Re: [PATCH-e2fsprogs] filefrag: fix fm_start in filefrag

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Date:	Wed, 03 Jun 2009 15:37:24 -0600
From:	Andreas Dilger <adilger@....com>
To:	Peng Tao <bergwolf@...il.com>
Cc:	linux-ext4@...r.kernel.org, tytso@....edu
Subject: Re: [PATCH-e2fsprogs] filefrag: fix fm_start in filefrag_fiemap	loop

On Jun 03, 2009  09:21 +0800, Peng Tao wrote:
> When used with -v and the targeted file has more than 144
> extents(double of the length of fm_extents array provided by buf),
> filefrag_fiemap loops and calls fiemap ioctl() multiple times to
> calculate the actual number of extents in a file. Each call to fiemap
> ioctl() uses  fm_start as the starting logical offset. The patch fixes
> fm_start in each loop( except for the first one) and makes the extent
> calculation correct for files with more that 144 extents.
> 
> To produce the problem, first run filefrag -v on a highly fragmented
> file. Then change the buf size in filefrag_fiemap to make it large
> enough to have all the extent mapped in a single loop and run filefrag
> -v after recompiling. The former will produce a much smaller extent
> count because of the false fm_start used in the loop. And the two will
> produce different extent output since the 145th extent.
> 
> BTW, should I resend the patch to include the above explanation?

Yes, please do.  I verified that the code as modified (just '=') is also
present in the Lustre fiemap patch, so I'm not sure how it got += there.
Possibly from an older version of the patch.

> > On Jun 03, 2009  02:03 +0800, Peng Tao wrote:
> >> Signed-off-by: Peng Tao <bergwolf@...il.com>
> >> ---
> >>  misc/filefrag.c |    2 +-
> >>  1 files changed, 1 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/misc/filefrag.c b/misc/filefrag.c
> >> index 6bea2cd..7786646 100644
> >> --- a/misc/filefrag.c
> >> +++ b/misc/filefrag.c
> >> @@ -239,7 +239,7 @@ int filefrag_fiemap(int fd, int blk_shift, int *num_extents)
> >>                       n++;
> >>               }
> >>
> >> -             fiemap->fm_start += fm_ext[i-1].fe_logical +
> >> +             fiemap->fm_start = fm_ext[i-1].fe_logical +
> >>                                                       fm_ext[i-1].fe_length;
> >>       } while (last == 0);
> >>
> >> --
> >> 1.6.2.GIT

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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