Re: fragmentation && blocks "realloc"

[Jan Koss <kossjan@gmail.com>]

In fact, I expected yes for the first abbility and no for the second :)

Now code looks like:
bh = sb_bread(sb, oldblock);
if (!bh)
  goto err;
bh->b_blocknr = newblk;				
mark_buffer_dirty (bh);
unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);

Let's suppose such test case:
after situation, which I described in the first email,
user resize file and new size 5 blocks,
and there are no free blocks except 2 blocks which we deallocated in
the frist email,
so we have to allocate them.

When I reproduced this test case, I got such messages from kernel:
__find_get_block_slow failed block=oldblock...

So as I can see I missed something in "art of changing b_blocknr".

Error in __find_get_block_slow may happen only if all buffers on page mapped.

May be this is because of buffer_head change b_blocknr, but didn't
change b_this_page?

On 1/20/06, Anton Altaparmakov <aia21@cam.ac.uk> wrote:
> On Fri, 2006-01-20 at 14:47 +0300, Jan Koss wrote:
> > Hello.
> >
> > Let's suppose that we have file which consist of two blocks
> > and user resizing file and now we need 4 blocks.
> >
> > Near this two blocks there are no 2 free blocks,
> > and instead of allocating 2 additional blocks somewhere,
> > I want allocate chunk of 4 blocks.
> >
> > The main problem is choose way of invalidate "old" blocks and copy
> > data to new buffers,
> >
> > how it possible on linux?
> >
> > something like
> > struct buffer_head *oldbh, *newbh;
> > memcpy(newbh->b_data, oldbh->b_data);
> > block_invalidatepage(oldbh->b_this_page,...)
>
> No need to invalidate or copy anything as long as you are working inside
> a file system driver and those buffers are attached to page cache of a
> file.
>
> > or it is possible just change b_blocknr?
>
> Yes, just change b_blocknr, and mark the buffer dirty so it gets written
> out to the new location or indeed you can do the write (or submission
> thereof) yourself if you want.
>
> Note since you are effectively "allocating" the buffer(s), after you
> have done the block allocation on your file system and updated
> bh->b_blocknr, you need to call unmap_underlying_metadata(bh->b_dev,
> bh->b_blocknr); for each block before you write it out or your write
> could get trampled on by a different write.
>
> And of course do not forget to deallocate the two blocks you just freed
> in your fs...  (-:
>
> Best regards,
>
>         Anton
> --
> Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
> Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
> Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
> WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/
>
>