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

[Matthew Wilcox]

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?