On 9.11.2017 19:53, Josef Bacik wrote:
> From: Josef Bacik <jbacik@xxxxxx>
>
> Since we're allocating under atomic we could every easily enomem, so if
> that's the case and we can block then loop around and try to allocate
> the prealloc not under a lock.
>
> We also saw this happen during try_to_release_page in production, in
> which case it's completely valid to return ENOMEM so we can tell
> try_to_release_page that we can't release this page.
>
> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> ---
> fs/btrfs/extent_io.c | 26 ++++++++++++++++++++++----
> 1 file changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index dd941885b9c3..6d1de1a81dc8 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -590,8 +590,9 @@ static int __clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
> struct extent_state *prealloc = NULL;
> struct rb_node *node;
> u64 last_end;
> - int err;
> + int err = 0;
> int clear = 0;
> + bool need_prealloc = false;
>
> btrfs_debug_check_extent_io_range(tree, start, end);
>
> @@ -614,6 +615,9 @@ static int __clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
> * If we end up needing a new extent state we allocate it later.
> */
> prealloc = alloc_extent_state(mask);
> + if (!prealloc && need_prealloc)
> + return -ENOMEM;
> + need_prealloc = false;
> }
>
> spin_lock(&tree->lock);
> @@ -673,7 +677,14 @@ static int __clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
>
> if (state->start < start) {
> prealloc = alloc_extent_state_atomic(prealloc);
> - BUG_ON(!prealloc);
> + if (!prealloc) {
> + if (gfpflags_allow_blocking(mask)) {
> + need_prealloc = true;
> + goto again;
At this point we already hold spin_lock(&tree->lock); so when we go to
again: directly we will deadlock. At the very least you'd want to unlock
the tree->lock spinlock.
In any case I hate how this function is structured. Can't we just make a
GFP_NOFAIL allocation for prealloc without if the gfp mask allows
holding the lock and ensure we alway have 1 preallocated extent_state
even if we don't need it when we can. So when we go into one of the
branches which require a prealloc if we don't have it then we know there
was no way to get it upfront and just return enomem straight away?
> + }
> + err = -ENOMEM;
> + goto out;
> + }
> err = split_state(tree, state, prealloc, start);
> if (err)
> extent_io_tree_panic(tree, err);
> @@ -696,7 +707,14 @@ static int __clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
> */
> if (state->start <= end && state->end > end) {
> prealloc = alloc_extent_state_atomic(prealloc);
> - BUG_ON(!prealloc);
> + if (!prealloc) {
> + if (gfpflags_allow_blocking(mask)) {
> + need_prealloc = true;
> + goto again;
> + }
> + err = -ENOMEM;
> + goto out;
> + }
> err = split_state(tree, state, prealloc, end + 1);
> if (err)
> extent_io_tree_panic(tree, err);
> @@ -731,7 +749,7 @@ static int __clear_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
> if (prealloc)
> free_extent_state(prealloc);
>
> - return 0;
> + return err;
>
> }
>
>
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