[Ocfs2-devel] FIEMAP problem

[David Weber <wb@munzinger.de>]

Am Mittwoch, 7. August 2013, 22:07:19 schrieb Jeff Liu:
> On 08/07/2013 05:17 PM, David Weber wrote:
> > Hi,
> > 
> > We are trying to use OCFS2 as VM storage. After running into problems with
> > qemu's disk_mirror feature we now think there could be a problem with the
> > FIEMAP ioctl in OCFS2.
> > 
> > As far as I understand the situation looks like this:
> > Qemu inquiries the FS if the given section of the image is already
> > allocated via the FIEMAP ioctl [1]
> > It especially checks if fm_mapped_extents is greater 0.
> > OCFS2 reports on sections bigger 1048576 there would be 0 mapped_extents
> > which is wrong.
> > 
> > I extended a userspace FIEMAP util [2] a bit to specify the start and
> > length parameter [3] as an easier testcase.
> > 
> > When we create a big file which has no holes
> > dd if=/dev/urandom of=/mnt/kvm-images/urandom.img bs=1M count=1000
> > 
> > We get on lower sections the expected output:
> > ./a.out /mnt/kvm-images/urandom.img 10000 10
> > start: 2710, length: a
> > File /mnt/kvm-images/urandom.img has 1 extents:
> > #       Logical          Physical         Length           Flags
> > 0:      0000000000000000 0000004ca3f00000 000000000be00000 0000
> > 
> > But on sections >= 1048576 it reports there wouldn't be any extents which
> > is as far as I understand wrong:
> > ./a.out /mnt/kvm-images/urandom.img 1048576 10
> > start: 100000, length: a
> > File /mnt/kvm-images/urandom.img has 0 extents:
> > #       Logical          Physical         Length           Flags
> 
> Thanks for your report, looks this problem has existed over years.
> As a quick response, could you please try the below fix?

Thank you very much! This solved the problems with qemu.

I found a fiemap-tester util[1] in the xfstests project and it runs fine on 
OCFS2 with 4K cluster size but fails with 1M. I have however no idea if this 
is a severe problem.

# gcc -DHAVE_FALLOCATE=1 -o fiemap-tester fiemap-tester.c 
# ./fiemap-tester /mnt/kvm-images/fiemap_test
Starting infinite run, if you don't see any output then its working properly.
HEY FS PERSON: your fs is weird.  I specifically wanted a
hole and you allocated a block anyway.  FIBMAP confirms that
you allocated a block, and the block is filled with 0's so
everything is kosher, but you still allocated a block when
didn't need to.  This may or may not be what you wanted,
which is why I'm only printing this message once, in case
you didn't do it on purpose. This was at block 0.
ERROR: preallocated extent is not marked with FIEMAP_EXTENT_UNWRITTEN: 0
map is 
'HDHPHHDDHPHPHPHDDHHPPDDPPPHHHPDDDPDHHHHDDDPPHPPPDPHHPPDPPHHDDPDPPHDHPDDDDPDPPDPHDDPPDDPPHDDPDHHHDDPDHPHPDPPDDHPHPPHDPHPHDDHDPDPDHDHPDDPHPPPHDPPDPDDHPHDDPPHPDHPPHPPHPHHPHDHPPDDPHDHHPPHPPDHPHPHDHPPDDDDPHHHPPPHHHDDDDPDPDDPPPHPHDPPPHDPDPHDDHPPPDPDHPHHPHDHHDHPDPHDDPPHDPPDDPDDPPDHPPDPDHHPHDHPPHDDHDPHPPPDHPDDDHDDHDPPHHDDPPDPDDHDHHPHDPHHPPPDPPDHDHHPPHDPHDPPHDPHHPPP'
logical: [       0..     255] phys: 132160512..132160767 flags: 0x000 tot: 256


[1] http://oss.sgi.com/cgi-bin/gitweb.cgi?p=xfs/cmds/xfstests.git;a=blob_plain;f=src/fiemap-tester.c;hb=HEAD


> 
> From: Jie Liu <jeff.liu@oracle.com>
> 
> Call fiemap ioctl(2) with given start offset as well as an desired
> mapping range should show extents if possible.  However, we calculate
> the end offset of mapping via 'mapping_end -= cpos' before iterating
> the extent records which would cause problems, e.g,
> 
> Cluster size 4096:
> debugfs.ocfs2 1.6.3
>         Block Size Bits: 12   Cluster Size Bits: 12
> 
> The extended fiemap test utility From David:
> https://gist.github.com/anonymous/6172331
> 
> # dd if=/dev/urandom of=/ocfs2/test_file bs=1M count=1000
> # ./fiemap /ocfs2/test_file 4096 10
> start: 4096, length: 10
> File /ocfs2/test_file has 0 extents:
> #	Logical          Physical         Length           Flags
> 	^^^^^ <-- No extents
> 
> In this case, at ocfs2_fiemap(): cpos == mapping_end == 1. Hence the
> loop of searching extent records was not executed at all.
> 
> This patch remove the in question 'mapping_end -= cpos', and loops
> until the cpos is larger than the mapping_end instead.
> 
> # ./fiemap /ocfs2/test_file 4096 10
> start: 4096, length: 10
> File /ocfs2/test_file has 1 extents:
> #	Logical          Physical         Length           Flags
> 0:	0000000000000000 0000000056a01000 0000000006a00000 0000
> 
> Reported-by: David Weber <wb@munzinger.de>
> Cc: Mark Fashen <mfasheh@suse.de>
> Cc: Joel Becker <jlbec@evilplan.org>
> Signed-off-by: Jie Liu <jeff.liu@oracle.com>
> ---
>  fs/ocfs2/extent_map.c |    1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/fs/ocfs2/extent_map.c b/fs/ocfs2/extent_map.c
> index 2487116..8460647 100644
> --- a/fs/ocfs2/extent_map.c
> +++ b/fs/ocfs2/extent_map.c
> @@ -781,7 +781,6 @@ int ocfs2_fiemap(struct inode *inode, struct
> fiemap_extent_info *fieinfo, cpos = map_start >> osb->s_clustersize_bits;
>  	mapping_end = ocfs2_clusters_for_bytes(inode->i_sb,
>  					       map_start + map_len);
> -	mapping_end -= cpos;
>  	is_last = 0;
>  	while (cpos < mapping_end && !is_last) {
>  		u32 fe_flags;
> 
> > We're running linux-3.11-rc4 plus the following patches:
> > [PATCH V2] ocfs2: update inode size after zeroed the hole
> > [PATCH RESEND] ocfs2: fix NULL pointer dereference in
> > ocfs2_duplicate_clusters_by_page
> > NULL pointer dereference at	ocfs2_dir_foreach_blk_id
> > [patch v3] ocfs2: ocfs2: fix recent memory corruption bug
> > 
> > o2info --volinfo  /dev/drbd0
> > 
> >        Label: kvm-images
> >        
> >         UUID: BE7C101466AD4F2196A849C7A6031263
> >   
> >   Block Size: 4096
> > 
> > Cluster Size: 1048576
> > 
> >   Node Slots: 8
> >   
> >     Features: backup-super strict-journal-super sparse extended-slotmap
> >     Features: inline-data xattr indexed-dirs refcount discontig-bg
> >     unwritten
> > 
> > Thanks in advance!
> > 
> > Cheers,
> > David
> > 
> > 
> > [1]
> > http://git.qemu.org/?p=qemu.git;a=blob;f=block/raw-posix.c;h=ba721d3f5bd9
> > 8a6b62791c2e20dbf2894021ad76;hb=HEAD#l1087
> > 
> > [2]
> > http://smackerelofopinion.blogspot.de/2010/01/using-fiemap-ioctl-to-get-f
> > ile-extents.html
> > 
> > [3] https://gist.github.com/anonymous/6172331
> > 
> > 
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