linux-kernel - Re: [PATCH] btrfs: remove BUG

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Date:   Wed, 18 Dec 2019 17:31:18 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Aditya Pakki <pakki001@....edu>
Cc:     kjlu@....edu, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] btrfs: remove BUG_ON used as assertions

On Sun, Dec 15, 2019 at 11:12:36AM -0600, Aditya Pakki wrote:
> alloc_extent_state_atomic() allocates extents via GFP_ATOMIC flag
> and cannot fail.

Here you say it cannot fail (without a proof)

> There are multiple invocations of BUG_ON on the
> return value to check for failure. The patch replaces certain
> invocations of BUG_ON by returning the error upstream.
> 
> Signed-off-by: Aditya Pakki <pakki001@....edu>
> ---
>  fs/btrfs/extent_io.c | 20 ++++++++++++++++----
>  1 file changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index eb8bd0258360..e72e5a333e71 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -989,7 +989,10 @@ __set_extent_bit(struct extent_io_tree *tree, u64 start, u64 end,
>  	node = tree_search_for_insert(tree, start, &p, &parent);
>  	if (!node) {
>  		prealloc = alloc_extent_state_atomic(prealloc);
> -		BUG_ON(!prealloc);
> +		if (!prealloc) {
> +			err = -ENOMEM;
> +			goto out;

And add error handling when it fails, which is in contradiction but
anyway, replacing BUG_ON with error checks requires auditing the whole
call chain up. We've had attempts to do this change already, none of them
was correct so unfortunatelly the BUG_ON needs to stay to avoid errors
being silently ignored.