Re: [PATCH] ext4: Remove extent tree purging from ext4_da_page_release_reservation()

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Wed, Jul 24, 2013 at 10:05:36PM +0200, Jan Kara wrote:
>   Hi Zheng,
> 
> On Fri 19-07-13 08:44:39, Zheng Liu wrote:
> > On Thu, Jul 18, 2013 at 12:10:15AM +0200, Jan Kara wrote:
> > > ext4_da_page_release_reservation() gets called from
> > > ext4_da_invalidatepage(). This function is used when we are truncating
> > > page cache for punch hole or truncate operations. In either case these
> > > operations take care of removing extents from the extent tree. This is
> > > more efficient and the code in ext4_da_page_release_reservation() is
> > > actually buggy anyway. So just remove it.
> > 
> > I remember that I try to remove the entry from extent status tree here
> > because at the end of this function it tries to relase the reserved
> > space for delalloc.  For 4k block we can simply release it because
> > ->s_cluster_ratio == 1.  But when bigalloc is enabled, we need to
> > determine whether we can release the reserved space according to the
> > result of ext4_find_delalloc_cluster() as the comment described.  If we
> > don't remove the entry from extent status tree here, we could lost some
> > spaces that could be reused by other files.  If I remember correctly, I
> > have hitted a warning message when I run xfstests to test it.  These
> > days I try to trigger it using xfstests but I failed.  Have you seen a
> > prblem that is caused by this code?  Maybe we need to refactor out the
> > code and release the reserved space outside this function.
>   Ah, I see. No, I didn't observe any problem due to this code, I just
> didn't understand why is it there. Also when blocksize < pagesize, the code
> is wrong because delayed buffers to release need not be contiguous so
> ext4_es_remove_extent(inode, lblk, to_release) may not free all the buffers
> we want. But subsequent extent tree truncation in ext4_ext_truncate() hides
> this problem.
> 
> So I think we might just change the condition:
> 
> if (to_release) {
> 
> to
> 
> if (to_release && sbi->s_cluster_ratio > 1) {
> 
> and add explanatory comment why cluster_ratio > 1 needs the truncation and
> other cases don't. It will also save some needlessly burned CPU cycles
> spent when manipulating extent tree.

Yes, thanks for pointing it out.  I attach a patch below.  Could you
please review it?

Thanks,
                                                - Zheng

Subject: [PATCH 1/2] ext4: remove the entry from es tree when bigalloc is enabled

From: Jan Kara <jack@suse.cz>

Now in ext4_da_page_release_reservation() we remove the entry from es
tree if to_release != 0.  But there are two issues.  One is that it is
wrong when blocksize != pagesize, another is that we don't need to do
this if ->s_cluster_ratio == 1 because we will remove the entry in
ext4_truncate/ext4_punch_hole.  Here we need to do this just because
when ->s_cluster_ratio > 1 we will determine whether we can release
the reserved space according to ext4_find_delalloc_cluster().

This commit tries to fix these problems.  Now we remove the entry from
es tree only if ->s_cluster_ratio > 1.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
---
 fs/ext4/inode.c |   10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index ba33c67..e0c8623 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1387,7 +1387,15 @@ static void ext4_da_page_release_reservation(struct page *page,
 		curr_off = next_off;
 	} while ((bh = bh->b_this_page) != head);
 
-	if (to_release) {
+	/*
+	 * Here we need to remove the entry from es tree because when bigalloc
+	 * is enabled we need to determine whether we can release the reserved
+	 * space according to the result of ext4_find_delalloc_cluster().
+	 *
+	 * If bigalloc is disabled, we don't need to do this here because these
+	 * extries in es tree will be removed in ext4_truncate/ext4_punch_hole.
+	 */
+	if (sbi->s_cluster_ratio > 1 && to_release) {
 		lblk = page->index << (PAGE_CACHE_SHIFT - inode->i_blkbits);
 		ext4_es_remove_extent(inode, lblk, to_release);
 	}
-- 
1.7.9.7