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

[mawupeng <mawupeng1@huawei.com>]

On 2024/2/28 10:10, Matthew Wilcox wrote:
> On Wed, Feb 28, 2024 at 09:55:24AM +0800, 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).
>>
>> just a device fd with device-specify mmap which use remap_pfn_range to assign memory.
> 
> But what meaning do you want MAP_PRIVATE of this fd to have?  Does it
> make sense to permit this, or should you rather just return -EINVAL if
> somebody tries to mmap() with MAP_PRIVATE set?

I think return -EINVAL if somebody tries to mmap() with MAP_PRIVATE and MAP_MAYWRITE is reasonable to me.

Read to this pfnmap vma will not trigger fault.

>