Date:	Tue, 4 Dec 2012 18:45:46 +0800
From:	Yongqiang Yang <xiaoqiangnk@...il.com>
To:	Darren Hart <dvhart@...radead.org>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [e2fsprogs] initdir: Writing inode after the initial write?

Hi,

If original images are ext4 format, this can be done by writing the
image to a new device and resizing the new device via resizefs.

Yongqiang,
Thanks,

On Tue, Dec 4, 2012 at 3:46 AM, Darren Hart <dvhart@...radead.org> wrote:
> On 12/01/2012 11:31 AM, Andreas Dilger wrote:
>> 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.
>
>
> Ted, can you confirm?
>
>
>> 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[*].
>
>
> I just didn't know about it and it didn't come up in my polling :-)
> (which would have been more fruitful had I done some of that here).
>
>
>> 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
>
>
> This is really promising. I've tweaked it a bit to use the basename and
> cd into the directories as they are traversed by find so it doesn't try
> and create filenames like "/dir1/hello.txt" in the root directory.
>
>         #!/bin/sh
>         SRCDIR=$1
>         DEVICE=$2
>
>         {
>                 find $SRCDIR | while read FILE; do
>                         #TGT=${FILE#$SRCDIR}
>                         TGT=$(basename ${FILE#$SRCDIR})
>
>                         # Skip the root dir
>                         if [ -z "$TGT" ]; then
>                                 continue
>                         fi
>
>                         case $(stat -c "%F" $FILE) in
>                         "directory")
>                                 echo "mkdir $TGT"
>                                 echo "cd $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 $TGT b $DEVNO"
>                                 ;;
>                         "character special file")
>                                 DEVNO=$(stat -c "%t %T" $FILE)
>                                 echo "mknod $TGT c $DEVNO"
>                                 ;;
>                         *)
>                                 echo "Unknown file $FILE" 1>&2
>                                 ;;
>                         esac
>                 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.
>
>
> It needs symlink as you said, but I can relatively easily migrate my
> code for that in mke2fs to debugfs.
>
> Still needs permissions and such. Is that done with "modify_inode" ? If
> so, how do I specify the new contents?
>
> I need to look into how to detect and support hard links.
>
>
>> 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.
>
>
> Yes, this seems to work just fine.
>
>
>>> 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.
>
>
> I plan to update this as I go... although I'm going to have much less to
> do if I use the debugfs approach. ;-)
>
> I wonder if it would make sense to integrate the debugfs functionality
> into libext2fs and enable both debugfs and mke2fs to use the same common
> code. I think the "-r initialdir" option would still be nice to have for
> mke2fs, and does make it more consistent with other FSs in this feature.
>
>
>>
>>> 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.
>
>
> OK, well it could also be done with ftw to be more portable, but I guess
> it's still marked obsolete in POSIX.1-2008 :/
>
> Similar functionality could be implemented relatively easily.
>
>
>>
>>> 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.
>
>
> I should be able to compare with debugfs "write" and see what the
> difference is.
>
>
>>
>>> 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.
>
> Not at all! I appreciate the tip. And it hasn't been wasted time, I've
> learned quite a bit, and as I said above, perhaps the debugfs copies and
> such can be pushed into libext2fs and used in both. ext2fs_mkdir()
> exists after all, why not ext2fs_mksymlink(), ext2fs_mknod() and
> ext2fs_writefile() ?
>
> Thanks a lot for the insight, exactly what I needed!
>
> --
> Darren
>
>>
>> [*] 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
>>>>> --
>>>>> To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
>>>>> the body of a message to majordomo@...r.kernel.org
>>>>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>>>>
>>>>
>>>> Cheers, Andreas
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> --
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>>>>
>>> --
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>>
>>
>> Cheers, Andreas
>>
>>
>>
>>
>>
>> --
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>>
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--
Best Wishes
Yongqiang Yang
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