[PATCH 2.6.13] clear_buffer_uptodate() in discard_buffer()

[Eric Sandeen <sandeen@sgi.com>]

I was tracking down a problem in XFS, with this testcase:

(where blocksize = 1/4 page size)

seek out 4 blocks
write 2 blocks (EOF now at 6 blocks)
truncate back to 4 blocks + a few bytes
truncate out to 6 blocks

This should wind up with:

4 blocks of hole | 1 block of data | 1 block of hole

but instead gave:

4 blocks of hole | 2 blocks of data

The last extending truncate, which should lead to a hole of 1 filesystem block 
at the end of the file, wound up getting allocated as an extent.  I tracked 
this down to the first truncate back, where we called discard_buffer() on the 
buffer head covering the last block in the file.  discard_buffer() clears 
several bh state flags, and nulls b_bdev, but it does NOT clear the uptodate flag.

XFS later comes along and allocates blocks for the file (due to a periodic 
sync, in the simplest case), and finds that the buffer_head over the last block 
before EOF is Uptodate, and allocates a block where there should be a hole.

I submit this patch with some hesitation, because a few years ago, akpm 
committed this changeset:

http://linux.bkbits.net:8080/linux-2.6/cset@1.373.70.7?nav=index.html|src/|src/fs|related/fs/buffer.c

which specifically -removed- the clearing of BH_Uptodate in discard_buffer().

Andrew had some concerns about clearing the bh uptodate flag without possibly 
changing the page state, but didn't otherwise remember the details. :)

However, when we truncate down & call discard_buffer, this buffer head is now 
out past EOF, and any other buffers also past EOF on this page have no state at 
all; they also are in their initialized state from alloc_page_buffers with 0 
b_state, and NULL b_bdev, etc.  So by clearing Uptodate on this truncated 
buffer, it simply joins it's friends at the end of the page.

Thanks,

-Eric

Signed-off-by: Eric Sandeen <sandeen.sgi.com>

--- a/fs/buffer.c	2005-09-14 15:54:19.000000000 -0500
+++ b/fs/buffer.c	2005-09-14 15:53:52.000000000 -0500
@@ -1549,6 +1549,7 @@
 	lock_buffer(bh);
 	clear_buffer_dirty(bh);
 	bh->b_bdev = NULL;
+	clear_buffer_uptodate(bh);
 	clear_buffer_mapped(bh);
 	clear_buffer_req(bh);
 	clear_buffer_new(bh);