On Feb 21, 2013, Alexandre Oliva <oliva@xxxxxxx> wrote:
> What I saw in that function also happens to explain why in some cases I
> see filesystems allocate a huge number of chunks that remain unused
> (leading to the scenario above, of not having more chunks to allocate).
> It happens for data and metadata, but not necessarily both. I'm
> guessing some thread sets the force_alloc flag on the corresponding
> space_info, and then several threads trying to get disk space end up
> attempting to allocate a new chunk concurrently. All of them will see
> the force_alloc flag and bump their local copy of force up to the level
> they see first, and they won't clear it even if another thread succeeds
> in allocating a chunk, thus clearing the force flag. Then each thread
> that observed the force flag will, on its turn, force the allocation of
> a new chunk. And any threads that come in while it does that will see
> the force flag still set and pick it up, and so on. This sounds like a
> problem to me, but... what should the correct behavior be? Clear
> force_flag once we copy it to a local force? Reset force to the
> incoming value on every loop?
I think a slight variant of the following makes the most sense, so I
implemented it in the patch below.
> Set the flag to our incoming force if we have it at first, clear our
> local flag, and move it from the space_info when we determined that we
> are the thread that's going to perform the allocation?
From: Alexandre Oliva <oliva@xxxxxxx>
btrfs: consume force_alloc in the first thread to chunk_alloc
Even if multiple threads in do_chunk_alloc look at force_alloc and see
a force flag, it suffices that one of them consumes the flag. Arrange
for an incoming force argument to make to force_alloc in case of
concurrent calls, so that it is used only by the first thread to get
to allocation after the initial request.
Signed-off-by: Alexandre Oliva <oliva@xxxxxxx>
---
fs/btrfs/extent-tree.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 6ee89d5..66283f7 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3574,8 +3574,12 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
again:
spin_lock(&space_info->lock);
+
+ /* Bring force_alloc to force and tentatively consume it. */
if (force < space_info->force_alloc)
force = space_info->force_alloc;
+ space_info->force_alloc = CHUNK_ALLOC_NO_FORCE;
+
if (space_info->full) {
spin_unlock(&space_info->lock);
return 0;
@@ -3586,6 +3590,10 @@ again:
return 0;
} else if (space_info->chunk_alloc) {
wait_for_alloc = 1;
+ /* Reset force_alloc so that it's consumed by the
+ first thread that completes the allocation. */
+ space_info->force_alloc = force;
+ force = CHUNK_ALLOC_NO_FORCE;
} else {
space_info->chunk_alloc = 1;
}
--
Alexandre Oliva, freedom fighter http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/ FSF Latin America board member
Free Software Evangelist Red Hat Brazil Compiler Engineer
