Re: [PATCH v2 6/7] mm: Batch around can_change_pte_writable()

[Dev Jain <dev.jain@arm.com>]

On 29/04/25 7:27 pm, Lorenzo Stoakes wrote:
> On Tue, Apr 29, 2025 at 11:27:43AM +0200, David Hildenbrand wrote:
>> 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.
> 
> This makes me concerned about the stability of this series as a whole...
> 
>>
>> If that test case does not detect the issue you're introducing, we should
>> look into adding a test case that detects it.
> 
> There are 25 tests that fail for the cow self-test with this series
> applied:
> 
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with base page
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (16 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (16 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (16 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (32 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (32 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (32 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (64 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (64 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (64 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (128 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (128 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (128 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (256 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (256 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (256 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (512 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (512 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (512 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (1024 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (1024 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (1024 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with PTE-mapped THP (2048 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with single PTE of THP (2048 kB)
> # [RUN] vmsplice() + unmap in child with mprotect() optimization ... with partially shared THP (2048 kB)
> 
> 
> Dev, please take a little more time to test your series :) the current
> patch set doesn't compile and needs fixes applied to do so, and we're at
> v2, and you've clearly not run self-tests as these also fail.
> 
> Please ensure you do a smoke test and check compilation before sending out,
> as well as running self tests also.

Apologies, I over-confidently skipped over selftests, and didn't build 
for x86 :( Shall take care.

> 
> Thanks, Lorenzo
> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>