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

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Message-ID: <d4fcf981-ea24-4892-9173-2c6f4046c7c1@arm.com>
Date: Thu, 13 Feb 2025 12:06:09 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Ryan Roberts <ryan.roberts@....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 2/7/25 16:29, Ryan Roberts wrote:
> 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.

Understood, thanks !