On Thu, Feb 21, 2013 at 06:15:49PM -0700, Alexandre Oliva wrote:
> 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;
So I understand what you are getting at, but I think you are doing it wrong. If
we're calling with CHUNK_ALLOC_FORCE, but somebody has already started to
allocate with CHUNK_ALLOC_NO_FORCE, we'll reset the space_info->force_alloc to
our original caller's CHUNK_ALLOC_FORCE. So we only really care about making
sure a chunk is actually allocated, instead of doing this flag shuffling we
should just do
if (space_info->chunk_alloc) {
spin_unlock(&space_info->lock);
wait_event(!space_info->chunk_alloc);
return 0;
}
and that way we don't allocate more chunks than normal. Thanks,
Josef
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