linux-kernel - Re: [PATCH v2 6/7] mm: Batch around can_change_pte

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Message-ID: <9a770aec-a020-4199-a53e-eddda657999d@redhat.com>
Date: Tue, 29 Apr 2025 11:27:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, akpm@...ux-foundation.org
Cc: ryan.roberts@....com, willy@...radead.org, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, catalin.marinas@....com, will@...nel.org,
 Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, vbabka@...e.cz,
 jannh@...gle.com, anshuman.khandual@....com, peterx@...hat.com,
 joey.gouly@....com, ioworker0@...il.com, baohua@...nel.org,
 kevin.brodsky@....com, quic_zhenhuah@...cinc.com,
 christophe.leroy@...roup.eu, yangyicong@...ilicon.com,
 linux-arm-kernel@...ts.infradead.org, namit@...are.com, hughd@...gle.com,
 yang@...amperecomputing.com, ziy@...dia.com
Subject: Re: [PATCH v2 6/7] mm: Batch around can_change_pte_writable()

On 29.04.25 11:19, David Hildenbrand wrote:
> 
>>    #include "internal.h"
>>    
>> -bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>> -			     pte_t pte)
>> +bool can_change_ptes_writable(struct vm_area_struct *vma, unsigned long addr,
>> +			      pte_t pte, struct folio *folio, unsigned int nr)
>>    {
>>    	struct page *page;
>>    
>> @@ -67,8 +67,9 @@ bool can_change_pte_writable(struct vm_area_struct *vma, unsigned long addr,
>>    		 * write-fault handler similarly would map them writable without
>>    		 * any additional checks while holding the PT lock.
>>    		 */
>> -		page = vm_normal_page(vma, addr, pte);
>> -		return page && PageAnon(page) && PageAnonExclusive(page);
>> +		if (!folio)
>> +			folio = vm_normal_folio(vma, addr, pte);
>> +		return folio_test_anon(folio) && !folio_maybe_mapped_shared(folio);
> 
> Oh no, now I spot it. That is horribly wrong.
> 
> Please understand first what you are doing.

Also, would expect that the cow.c selftest would catch that:

"vmsplice() + unmap in child with mprotect() optimization"

After fork() we have a R/O PTE in the parent. Our child then uses 
vmsplice() and unmaps the R/O PTE, meaning it is only left mapped by the 
parent.

ret = mprotect(mem, size, PROT_READ);
ret |= mprotect(mem, size, PROT_READ|PROT_WRITE);

should turn the PTE writable, although it shouldn't.

If that test case does not detect the issue you're introducing, we 
should look into adding a test case that detects it.

-- 
Cheers,

David / dhildenb