Re: [e2fsprogs] initdir: Writing inode after the initial write?

Andreas Dilger
2012-12-01T19:32:00Z

On 2012-11-30, at 10:08 PM, Darren Hart wrote:
> On 11/30/2012 08:23 PM, Andreas Dilger wrote:
>> On 2012-11-30, at 7:13 PM, Darren Hart wrote:
>>> I am working on creating some files after creating a filesystem in
>>> mke2fs. This is part of a larger project to add initial directory
>>> support to mke2fs.
>>
>> Maybe some background on what you are trying to do would help us to
>> understand the problem?
>
> Sure, a few are already aware, but I suppose some extra detail for
> the first post to this list is in order.
>
> I work on the Yocto Project, and this particular effort is part of
> improving our deployment tooling. Specifically, the part of the build
> process that creates the root filesystem.
>
> Most all filesystems have some mechanism to create prepopulated
> images without the need for root permissions. Many do this through
> a -r parameter to their corresponding mkfs.* tool. The exceptions to
> this are ext3 and ext4. Our current tooling relies on genext2fs and
> flipping some bits to "convert" the ext2 filesystem to ext3 and 4.
> Not ideal.
>
> After exploring options like libguestfs and finding them to be
> considerably heavy weight for what we are trying to accomplish, I
> discussed the possibility of adding an argument to mke2fs which would
> populate a newly formatted filesystem from a specified directory. Ted
> suggested a clean set of patches implementing this were likely to be
> accepted.

Hmm, I wonder if libext2fs can itself create extent-mapped files,
or if these files will be block-mapped? If they are small (< 1MB),
it is probably not a huge problem, but if your files are large it
may be that libext2fs also creates "ext2" files internally?

Maybe Ted can confirm whether that is true or not. At least I recall
that the block allocator inside libext2fs was horrible, and creating
large files was problematic.

I guess the other question is why you don't use debugfs to create
the directory tree and copy the files into your new filesystem?
It already has "mkdir", "mknod" and "write" commands for use, and
it is a one-line patch to alias "write" to "cp" for easier use[*].

Then, it just needs a debugfs script to build your directory tree
and copy files over. Possibly enhancing "cp" to call do_mknod() for
pipe/block/char devices would make this easier to use.

Something like the following, though it seems there isn't an "ln -s"
or "symlink" command for debugfs yet, that would need to be written.

#!/bin/bash
SRCDIR=$1
DEVICE=$2

{
find $SRCDIR | while read FILE; do
TGT=${FILE#$SRCDIR}
case $(stat -c "%F" $FILE) in
"directory")
echo "mkdir $TGT"
;;
"regular file")
echo "write $FILE $TGT"
;;
"symbolic link")
LINK_TGT=$(ls -l $FILE | sed -e 's/.*-> //')
echo "symlink $TGT $LINK_TGT"
;;
"block special file")
DEVNO=$(stat -c "%t %T" $FILE)
echo "mknod $F $DEVNO $TGT
;;
"character special file")
DEVNO=$(stat -c "%t %T" $FILE)
echo "mknod $TYPE $DEVNO $TGT
;;
*)
echo "Unknown file $FILE" 1>&2
;;
done
done
} | debugfs -w -f /dev/stdin $device

I would guess that implementing "symlink" support in debugfs will
be orders of magnitude less work, maintenance, and bugs than your
current patch.

This might be turned inside-out and just run a "find $SRCDIR" and
have the inner loop check the file type and call the appropriate
operation for it (mkdir, write/cp, mknod, symlink). Note that
"find" will return the directories first, so this should be OK to
just consume the lines as they are output by find.

> I don't have much filesystem experience - most of my experience is
> with core kernel mechanisms, ipc, locking, etc. - so I'm mostly
> hacking my way to some basic functionality before refactoring. The
> libext2fs library documentation gave me a good start, but I
> occasionally trip over things like the problem described below as
> there is no documentation for what I'm trying to do specifically
> (of course) and many of the required functions are only minimally
> documented, and sometimes only listed in the index.

Definitely, if the documentation is lacking and you've spent cycles
figuring something out, then a patch to improve the documentation is
most welcome.

> The specific instance below is the result of me trying to format and
> populate a filesystem image (in a file) from a root directory that looks like this:
>
> $ tree rootdir/
> rootdir/
> |-- dir1
> | |-- hello.lnk -> /hello.txt
> | `-- world.txt
> |-- hello.lnk -> /hello.txt
> |-- hello.txt
> |-- sda
> `-- ttyS0
>
> $ cat rootdir/hello.txt
> hello
>
> In mke2fs.c I setup the new getopt argument and call nftw() with a
> callback called init_dir_cb() which checks the file type and takes
> the appropriate action to duplicate each entry. The exact code is at:

To be honest, ntfw() will drag a bunch of bloat into e2fsprogs that
doesn't exist today, and isn't really portable.

> http://git.infradead.org/users/dvhart/e2fsprogs/blob/refs/heads/initialdir:/misc/mke2fs.c#l2319
>
> As described below, when I update the inode.i_size after the initial
> write and copying of the file content, the above cat command fails to
> output anything when run on the loop mounted filesystem. If I just
> hack in the i_size prior to writing the inode for the first time and
> don't update it after copying the file content, then the cat command
> succeeds as above on the loop mounted image.

It probably makes sense to understand what is broken here, whether
it is the library or the program. We definitely want to make sure
the API is usable and working correctly in any case.

> The commented out inode write is noted here:
>
> http://git.infradead.org/users/dvhart/e2fsprogs/blob/refs/heads/initialdir:/misc/mke2fs.c#l2462
>
> Does that help clarify the situation?
>
> What I'm looking for is some insight into what it is I am not
> understanding about the filesystem structures that causes this behavior.

I hate to put a downer on your current work, but I think that you
are adding something overly complex that only has a very limited
usefulness, and your time could be better spent elsewhere.

[*] add debugfs "cp" command as an alias to "write":

diff --git a/debugfs/debug_cmds.ct b/debugfs/debug_cmds.ct
index a799dd7..3789dcd 100644
--- a/debugfs/debug_cmds.ct
+++ b/debugfs/debug_cmds.ct
@@ -119,7 +119,7 @@ request do_undel, "Undelete file",
undelete, undel;

request do_write, "Copy a file from your native filesystem",
- write;
+ write, cp;

request do_dump, "Dump an inode out to a file",
dump_inode, dump;

> Thanks,
>
> Darren
>
>>
>> Cheers, Andreas
>>
>>> To make it easy for people to see what I'm working
>>> on, I've pushed my dev tree here:
>>>
>>> http://git.infradead.org/users/dvhart/e2fsprogs/shortlog/refs/heads/initialdir
>>>
>>> Note: the code is still just in the prototyping state. It is inelegant
>>> to say the least. The git tree will most definitely rebase. I'm trying
>>> to get it functional, once that is understand, I will refactor
>>> appropriately.
>>>
>>> I can create a simple directory structure and link in files and fast
>>> symlinks. I'm currently working on copying content from files in the
>>> initial directory. The process I'm using is as follows:
>>>
>>>
>>> ext2fs_new_inode(&ino)
>>> ext2fs_link()
>>>
>>> ext2fs_read_inode(ino, &inode)
>>> /* some initial inode setup */
>>> ext2fs_write_new_inode(ino, &inode)
>>>
>>> ext2fs_file_open2(&inode)
>>> ext2fs_write_file()
>>> ext2fs_file_close()
>>>
>>> inode.i_size = bytes_written
>>> ext2fs_write_inode()
>>>
>>> ext2fs_inode_alloc_stats2(ino)
>>>
>>>
>>> When I mount the image, the size for the file is correct, by catting it
>>> returns nothing. If I instead hack in the known size during the initial
>>> inode setup and drop the last ext2fs_write_inode() call, then the size
>>> is right and catting the file works as expected.
>>>
>>> Is it incorrect to write the inode more than once? If not, am I doing
>>> something that is somehow decoupling the block where the data was
>>> written from the inode associated with the file?
>>>
>>> Thanks,
>>>
>>> --
>>> Darren Hart
>>> Intel Open Source Technology Center
>>> Yocto Project - Technical Lead - Linux Kernel
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>>
>>
>> Cheers, Andreas
>>
>>
>>
>>
>>
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>>
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Cheers, Andreas