Re: [Question] CoW on VM_PFNMAP vma during write fault

[mawupeng]

On 2024/3/4 16:57, David Hildenbrand wrote:
> On 04.03.24 09:47, mawupeng wrote:
>> Hi Maintainers, kindly ping...
>>
>> On 2024/2/28 9:55, mawupeng wrote:
>>>
>>>
>>> On 2024/2/27 21:15, David Hildenbrand wrote:
>>>> On 27.02.24 14:00, David Hildenbrand wrote:
>>>>> On 27.02.24 13:28, Wupeng Ma wrote:
>>>>>> We find that a warn will be produced during our test, the detail log is
>>>>>> shown in the end.
>>>>>>
>>>>>> The core problem of this warn is that the first pfn of this pfnmap vma is
>>>>>> cleared during memory-failure. Digging into the source we find that this
>>>>>> problem can be triggered as following:
>>>>>>
>>>>>> // mmap with MAP_PRIVATE and specific fd which hook mmap
>>>>>> mmap(MAP_PRIVATE, fd)
>>>>>>      __mmap_region
>>>>>>        remap_pfn_range
>>>>>>        // set vma with pfnmap and the prot of pte is read only
>>>>>>      
>>>>>
>>>>> Okay, so we get a MAP_PRIVATE VM_PFNMAP I assume.
>>>>>
>>>>> What fd is that exactly? Often, we disallow private mappings in the
>>>>> mmap() callback (for a good reason).
>>
>> We found this problem in 5.10, Commit 9f78bf330a66 ("xsk: support use vaddr as ring") Fix this
>> problem during supporting vaddr by remap VM_PFNMAP by VM_MIXEDMAP. But other modules which
>> use remap_pfn_range may still have this problem.
> 
> I wrote a simple reproducer using MAP_PRIVATE of iouring queues on Friday.
> 
>>
>> It do seems wired for private mappings, What is the good reason?
> 
> I'm sure there are some use cases that require MAP_PRIVATE of such areas, and usually there is nothing wrong with that.

So MAP_PRIVATE for VM_PFNMAP area with write access is ok? What is the user case for this situation?

> 
> It's just that the PAT implementation incompatible.

PAT do have its problem.

> 
> I can submit a cleaned-up version of my patches.

Thanks

>