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

[David Hildenbrand <david@redhat.com>]

On 04.03.24 10:04, mawupeng wrote:
> 
> 
> 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?
I recall that MAP_PRIVATE /dev/mem mappings were required for some use 
cases. No details/ideas about other users, though.

Likely sufficient use case that people really thought about ways to get 
it working -- see vm_normal_page() :)

-- 
Cheers,

David / dhildenb