Re: [RFC][PATCH 7/9]ext4: Add the EXT4_IOC_FIEMAP_INO ioctl

[Akira Fujita <a-fujita@rs.jp.nec.com>]

Hi Ted,

Theodore Tso wrote:
> On Fri, Oct 31, 2008 at 06:05:47AM -0400, Christoph Hellwig wrote:
>>> I'll hack up a generic open_by_handle and then we can gather the
>>> reaction - it shouldn't be more than about one or two hundred lines of
>>> code.  Note that you really want an open by handle and not just inum for
>>> a defragmentation tool - without the generation you can easily run into
>>> races.
>
> I think having a generic open_by_handle is a Good Thing, but it
> actually isn't quite enough for defrag, because that brings up the
> question of how defrag can create the handle in the first place.
>
> In the case of Aryan's desire to get the list of files that were read
> during boot, it's pretty obvious how we can define an interface which
> would make available a set of file handles corresponding to the files
> that were opened during the boot process, and then that list of file
> handles can be saved to a file and used on the subsequent boot to do
> the readahead.   Fairly straight forward.
>
> In the case of the defrag situation, though, we need to step back and
> figure out what we are trying to do.  What the userspace program is
> apparently trying to do is to get the block extent maps used by all of
> the inodes in the block group.  The problem is we aren't opening the
> inodes by pathname, so we couldn't create a handle in the first place
> (since in order to create a handle, we need the containing directory).
>
> The bigger question is whether the defrag code is asking the right
> question in the first place.  The issue is that is that it currently
> assumes that in order to find the owner of a particular block (or more
> generally, extent), you should search the inodes in the block's
> blockgroup.  The problem is that for a very fragmented filesystem,
> most of the blocks' owners might not be in their block group.  In
> fact, over time, if you use defrag -f frequently, it will move blocks
> belonging to inodes in one block group to other block groups, which
> will make defrag -f's job even harder, and eventually impossible, for
> inodes belonging to other block groups.
>
>> Akira, can you please comment on these issues before going on?
>> I think the generation issue is a particularly important one if you
>> want to allow defrag by normal users.
>
> I'm not at all sure that it makes sense to allow "defrag -f" to be
> used by normal users.  The problem here is we're fighting multiple
> constraints.  First of all, we want to keep policy in the kernel to an
> absolute minimum, since debugging kernel code is a mess, and I don't
> think we want the complexity of a full-fledge defragger in the kernel.
> Secondly, though, if we are going to do this in userspace, we need to
> push a huge amount of information to the userspace defrag program,
> that immediately raises some very serious security issues, because we
> don't want to leak information unduly to random userspace programs.

All right.
I will change "defrag -f" to admit only root user.

>>> Btw, any reason the XFS approach of passing in *file descriptors* for both
>>> the inode to be defragmented and the "donor" inode doesn't work for you?
>
> I agree this is probably the better approach; it would certainly
> reduce the amount of new code that needs to be added to the kernel.
> Right now the "donor"/temporary inode is created and allocated in the
> kernel, and then the kernel decides whether or not the temporary inode
> is an improvement.  If we make the userspace code responsible for
> creating the temporary inode and then using fallocate() to allocate
> the new blocks, then userspace can call FIEMAP and decide whether it
> is an improvement.

There is no problem if it is only for normal defrag,
but I think fallocate is not enough for the other defrag mode (-r and -f)
because we can't specify physical block offset.
For example, in defrag -r, physical block offset of parent directory is
set to the goal for mballoc.
Also in defrag -f, physical block offset is used to allocate specified
range as well.

Do you mean that defrag -r and -f are unnecessary?
I think these options are advantages
if we compare ext4 defrag with other FS's defrag feature.

>
> P.S. I've been looking at ext4_defrag_partial(), and the page locking
> looks wrong.  The page is only locked if it it hasn't been read into
> memory yet?   And at the end of the function, we do this?
>
>        	      if (PageLocked(page))
> 			unlock_page(page);

In case defrag failed between write_begin() and write_end(),
we have to call unlock_page() because we've already have the page lock
with write_begin().
So unlock_page() is called in the end of ext4_defrag_partial() as error case.
Is there something wrong?

Regards,
Akira Fujita