linux-kernel - Re: [PATCH v3 10/10] mm: kvmalloc: Add non-blocking support for vmalloc

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Message-ID: <aN9tbS3RkEqm2tzP@tiehlicka>
Date: Fri, 3 Oct 2025 08:30:05 +0200
From: Michal Hocko <mhocko@...e.com>
To: "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 10/10] mm: kvmalloc: Add non-blocking support for
 vmalloc

On Wed 01-10-25 21:26:46, Uladzislau Rezki wrote:
> Extend __kvmalloc_node_noprof() to handle non-blocking GFP flags
> (GFP_NOWAIT and GFP_ATOMIC). Previously such flags were rejected,
> returning NULL. With this change:
> 
> - kvmalloc() can fall back to vmalloc() if non-blocking contexts;
> - for non-blocking allocations the VM_ALLOW_HUGE_VMAP option is
>   disabled, since the huge mapping path still contains might_sleep();
> - documentation update to reflect that GFP_NOWAIT and GFP_ATOMIC
>   are now supported.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>

Acked-by: Michal Hocko <mhocko@...e.com>
Thanks!

> ---
>  mm/slub.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 584a5ff1828b..3de0719e24e9 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7018,7 +7018,7 @@ static gfp_t kmalloc_gfp_adjust(gfp_t flags, size_t size)
>   * Uses kmalloc to get the memory but if the allocation fails then falls back
>   * to the vmalloc allocator. Use kvfree for freeing the memory.
>   *
> - * GFP_NOWAIT and GFP_ATOMIC are not supported, neither is the __GFP_NORETRY modifier.
> + * GFP_NOWAIT and GFP_ATOMIC are supported, the __GFP_NORETRY modifier is not.
>   * __GFP_RETRY_MAYFAIL is supported, and it should be used only if kmalloc is
>   * preferable to the vmalloc fallback, due to visible performance drawbacks.
>   *
> @@ -7027,6 +7027,7 @@ static gfp_t kmalloc_gfp_adjust(gfp_t flags, size_t size)
>  void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), unsigned long align,
>  			     gfp_t flags, int node)
>  {
> +	bool allow_block;
>  	void *ret;
>  
>  	/*
> @@ -7039,16 +7040,22 @@ void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), unsigned long align,
>  	if (ret || size <= PAGE_SIZE)
>  		return ret;
>  
> -	/* non-sleeping allocations are not supported by vmalloc */
> -	if (!gfpflags_allow_blocking(flags))
> -		return NULL;
> -
>  	/* Don't even allow crazy sizes */
>  	if (unlikely(size > INT_MAX)) {
>  		WARN_ON_ONCE(!(flags & __GFP_NOWARN));
>  		return NULL;
>  	}
>  
> +	/*
> +	 * For non-blocking the VM_ALLOW_HUGE_VMAP is not used
> +	 * because the huge-mapping path in vmalloc contains at
> +	 * least one might_sleep() call.
> +	 *
> +	 * TODO: Revise huge-mapping path to support non-blocking
> +	 * flags.
> +	 */
> +	allow_block = gfpflags_allow_blocking(flags);
> +
>  	/*
>  	 * kvmalloc() can always use VM_ALLOW_HUGE_VMAP,
>  	 * since the callers already cannot assume anything
> @@ -7056,7 +7063,7 @@ void *__kvmalloc_node_noprof(DECL_BUCKET_PARAMS(size, b), unsigned long align,
>  	 * protection games.
>  	 */
>  	return __vmalloc_node_range_noprof(size, align, VMALLOC_START, VMALLOC_END,
> -			flags, PAGE_KERNEL, VM_ALLOW_HUGE_VMAP,
> +			flags, PAGE_KERNEL, allow_block ? VM_ALLOW_HUGE_VMAP:0,
>  			node, __builtin_return_address(0));
>  }
>  EXPORT_SYMBOL(__kvmalloc_node_noprof);
> -- 
> 2.47.3

-- 
Michal Hocko
SUSE Labs