Re: [PATCH v2 3/3] mm/percpu: Avoid IO/FS reclaim in backing allocations

[Andrew Morton]

On Thu,  4 Jun 2026 19:31:01 +0800 Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:

> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> 
> Commit 9a5b183941b5 ("mm, percpu: do not consider sleepable
> allocations atomic") allows sleepable GFP_NOIO and GFP_NOFS percpu
> allocations to take pcpu_alloc_mutex.  This avoids premature allocation
> failures, but it also makes the mutex visible to callers from constrained
> IO/FS contexts.
> 
> Thread A calls pcpu_alloc_noprof() with GFP_KERNEL and takes
> pcpu_alloc_mutex. Since the internal allocation is not constrained by
> NOFS, it may enter FS reclaim while still holding pcpu_alloc_mutex,
> creating a dependency like: pcpu_alloc_mutex -> fs_reclaim -> FS lock
> 
> At the same time, Thread B may already hold an FS lock and then call
> pcpu_alloc_noprof() with GFP_NOFS. It will try to acquire
> pcpu_alloc_mutex and block, creating the reverse dependency:
> FS lock -> pcpu_alloc_mutex
> 
> This can still form a potential deadlock cycle.
> 
> Avoid the dependency by restricting percpu backing allocations to GFP_NOIO.
> The public allocation still uses the caller's GFP context to decide whether
> it may block, but the internal memory allocations performed while
> pcpu_alloc_mutex is held cannot recurse into IO or FS reclaim.
>
> ...
>
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1726,9 +1726,8 @@ static void pcpu_alloc_tag_free_hook(struct pcpu_chunk *chunk, int off, size_t s
>   * @gfp: allocation flags
>   *
>   * Allocate percpu area of @size bytes aligned at @align.  If @gfp doesn't
> - * contain %GFP_KERNEL, the allocation is atomic. If @gfp has __GFP_NOWARN
> - * then no warning will be triggered on invalid or failed allocation
> - * requests.
> + * allow blocking, the allocation is atomic. If @gfp has __GFP_NOWARN then no
> + * warning will be triggered on invalid or failed allocation requests.
>   *
>   * RETURNS:
>   * Percpu pointer to the allocated area on success, NULL on failure.
> @@ -1749,8 +1748,14 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
>  	size_t bits, bit_align;
>  
>  	gfp = current_gfp_context(gfp);
> -	/* whitelisted flags that can be passed to the backing allocators */
> -	pcpu_gfp = gfp & (GFP_KERNEL | __GFP_NORETRY | __GFP_NOWARN);
> +	/*
> +	 * Whitelisted flags that can be passed to the backing allocators.

We're supposed to say "allowlist".

> +	 * Backing allocations under pcpu_alloc_mutex must not recurse into
> +	 * IO/FS reclaim.  Otherwise a GFP_KERNEL caller holding the mutex can
> +	 * block on reclaim while a GFP_NOIO/NOFS caller holding an IO/FS lock
> +	 * waits for the same mutex.
> +	 */
> +	pcpu_gfp = gfp & (GFP_NOIO | __GFP_NORETRY | __GFP_NOWARN);

AI review
(https://sashiko.dev/#/patchset/20260604113101.89510-1-kaitao.cheng@xxxxxxxxx)
asked why we're currently removing __GFP_NOFAIL here.  There are
probably good reasons for this, but it would be good to describe them
in that comment.