Re: [PATCH v2 14/19] maple_tree: WARN_ON_ONCE when allocations fail

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 30 Jun 2026 15:08:38 -0400 "Liam R. Howlett (Oracle)" <liam@infradead.org> wrote:

> Allocations should never fail in the circumstances that are expected to
> occur.  Add checks in the code to ensure the circumstances are correctly
> set up by the user and warn if they are not.
> 
> Also add a warning on failure to allocate, which should never happen.
> 
> ...
>
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -5720,6 +5720,10 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
>  	if (likely(mas->node != MA_ERROR(-ENOMEM)))
>  		return false;
>  
> +	/* Allocations can fail, don't do this. */
> +	WARN_ON_ONCE(!gfpflags_allow_blocking(gfp) &&
> +		     mt_external_lock(mas->tree));
> +
>  	if (gfpflags_allow_blocking(gfp) && !mt_external_lock(mas->tree)) {
>  		mtree_unlock(mas->tree);
>  		mas_alloc_nodes(mas, gfp);
> @@ -5730,9 +5734,12 @@ bool mas_nomem(struct ma_state *mas, gfp_t gfp)
>  
>  	/*
>  	 * Return false on zero forward progress.  Partial allocations are kept
> -	 * so the retry path will attempt to get the rest.
> +	 * so the retry path will attempt to get the rest.  The failure should
> +	 * not happen as we try our best to reclaim.  The user would need an
> +	 * external lock with a non-blocking gfp in a low memory situation -
> +	 * which would have triggered the first warning in this function.
>  	 */
> -	if (!mas->sheaf && !mas->alloc)
> +	if (WARN_ON_ONCE(!mas->sheaf && !mas->alloc))
>  		return false;
>  

Can either of these warnings duplicate the effect of !__GFP_NOWARN?