From: Eric Sandeen 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. Signed-off-by: Eric Sandeen Signed-off-by: Andrew Morton --- fs/buffer.c | 1 + 1 files changed, 1 insertion(+) diff -puN fs/buffer.c~clear_buffer_uptodate-in-discard_buffer fs/buffer.c --- devel/fs/buffer.c~clear_buffer_uptodate-in-discard_buffer 2005-09-14 20:48:30.000000000 -0700 +++ devel-akpm/fs/buffer.c 2005-09-14 20:48:30.000000000 -0700 @@ -1549,6 +1549,7 @@ static inline void discard_buffer(struct 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); _