linux-kernel - Re: [PATCH v1 10/16] mm/vmalloc: Warn on improper use of vunmap

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Message-ID: <faa89fec-4b02-4844-859e-3b0e7f402d89@arm.com>
Date: Fri, 7 Feb 2025 10:59:53 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Anshuman Khandual <anshuman.khandual@....com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 Muchun Song <muchun.song@...ux.dev>,
 Pasha Tatashin <pasha.tatashin@...een.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Uladzislau Rezki <urezki@...il.com>, Christoph Hellwig <hch@...radead.org>,
 Mark Rutland <mark.rutland@....com>, Ard Biesheuvel <ardb@...nel.org>,
 Dev Jain <dev.jain@....com>, Alexandre Ghiti <alexghiti@...osinc.com>,
 Steve Capper <steve.capper@...aro.org>, Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 10/16] mm/vmalloc: Warn on improper use of
 vunmap_range()

On 07/02/2025 08:41, Anshuman Khandual wrote:
> On 2/5/25 20:39, 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 subsquently call vunmap_range() on a
> 
> s/subsquently/subsequently
> 
>> 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 jsut expected that callers will not do this.
> 
> s/jsut/just
> 
>>
>> However, it would be useful to know if this happened in future; let's
>> add a warning to cover the eventuality.
> 
> This is a reasonable check to prevent bad outcomes later.
> 
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>  mm/vmalloc.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index a6e7acebe9ad..fcdf67d5177a 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);
> Why not also include such checks in vunmap_p4d_range() and __vunmap_range_noflush()
> for corresponding P4D and PGD levels as well ?

The kernel does not support p4d or pgd leaf entries so there is nothing to check.

Although vunmap_p4d_range() does call p4d_clear_huge(). The function is a stub
and returns void (unlike p[mu]d_clear_huge()). I suspect we could just remove
p4d_clear_huge() entirely. But that would be a separate patch to mm tree I think.

For pgd, there isn't even an equivalent looking function.

Basically at those 2 levels, it's always a table.

Thanks,
Ryan