linux-kernel - Re: [PATCH v2 08/14] mm/vmalloc: Warn on improper use of vunmap

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Message-ID: <dd60e3d2-62ad-4295-8963-35bb1a94b006@arm.com>
Date: Thu, 20 Feb 2025 12:32:37 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 David Hildenbrand <david@...hat.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Alexandre Ghiti
 <alexghiti@...osinc.com>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 08/14] mm/vmalloc: Warn on improper use of
 vunmap_range()

On 2/17/25 19:38, Ryan Roberts wrote:
> A call to vmalloc_huge() may cause memory blocks to be mapped at pmd or
> pud level. But it is possible to subsequently call vunmap_range() on a
> sub-range of the mapped memory, which partially overlaps a pmd or pud.
> In this case, vmalloc unmaps the entire pmd or pud so that the
> no-overlapping portion is also unmapped. Clearly that would have a bad
> outcome, but it's not something that any callers do today as far as I
> can tell. So I guess it's just expected that callers will not do this.
> 
> However, it would be useful to know if this happened in future; let's
> add a warning to cover the eventuality.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>

LGTM and stands on its own independent of the series here.

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

> ---
>  mm/vmalloc.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 61981ee1c9d2..a7e34e6936d2 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -374,8 +374,10 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
>  		if (cleared || pmd_bad(*pmd))
>  			*mask |= PGTBL_PMD_MODIFIED;
>  
> -		if (cleared)
> +		if (cleared) {
> +			WARN_ON(next - addr < PMD_SIZE);
>  			continue;
> +		}
>  		if (pmd_none_or_clear_bad(pmd))
>  			continue;
>  		vunmap_pte_range(pmd, addr, next, mask);
> @@ -399,8 +401,10 @@ static void vunmap_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
>  		if (cleared || pud_bad(*pud))
>  			*mask |= PGTBL_PUD_MODIFIED;
>  
> -		if (cleared)
> +		if (cleared) {
> +			WARN_ON(next - addr < PUD_SIZE);
>  			continue;
> +		}
>  		if (pud_none_or_clear_bad(pud))
>  			continue;
>  		vunmap_pmd_range(pud, addr, next, mask);