linux-kernel - Re: [PATCH v3] mm/vmalloc: clarify why vmap_range

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Message-ID: <66068425-c9b2-475e-81ad-98f410d33b45@arm.com>
Date: Tue, 16 Dec 2025 09:46:37 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Brendan Jackman <jackmanb@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/vmalloc: clarify why vmap_range_noflush() might
 sleep



On 15/12/25 4:10 PM, Brendan Jackman wrote:
> The only reason vmap_range_noflush() can sleep is because of pagetable
> allocations.
> 
> The actual allocation mechanism is arch-specific so might_alloc()
> doesn't work here (what GFP flags would be used?). Hence, just add a
> comment.
> 
> Also note that this might do a TLB shootdown. This is not actually
> sleeping but it requires IRQs on for x86, and might_sleep() incidentally
> serves to detect violations of that too.
> 
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>

> ---
> Applies to mm-new.
> 
> Changes in v3:
> - Added note about TLB shootdowns and x86.
> - Since the comment no longer fit on one line anyway, also be a bit more
>   verbose in general.
> - Link to v2: https://lore.kernel.org/r/20251209-b4-vmalloc-might_alloc-v2-1-2462ba26ead1@google.com
> 
> Changes in v2:
> - Switched from might_alloc() to keeping might_sleep() + comment
> - Link to v1: https://lore.kernel.org/r/20251208-b4-vmalloc-might_alloc-v1-1-94a9bb8ecb08@google.com
> ---
>  mm/vmalloc.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 41dd01e8430c548167d6123f6ec66bf798f20eec..be4e5f2566a4490d45de29568df538b7c8515b01 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -305,6 +305,11 @@ static int vmap_range_noflush(unsigned long addr, unsigned long end,
>  	int err;
>  	pgtbl_mod_mask mask = 0;
>  
> +	/*
> +	 * Might allocate pagetables (for most archs a more precise annotation
> +	 * would be might_alloc(GFP_PGTABLE_KERNEL)). Also might shootdown TLB
> +	 * (requires IRQs enabled on x86).
> +	 */
>  	might_sleep();
>  	BUG_ON(addr >= end);
>  
> 
> ---
> base-commit: d0a24447990a9d8212bfb3a692d59efa74ce9f86
> change-id: 20251208-b4-vmalloc-might_alloc-754a791e4e10
> 
> Best regards,