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

[David Hildenbrand <david@redhat.com>]

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).

> // memset this memory with trigger fault
> handle_mm_fault
>    __handle_mm_fault
>      handle_pte_fault
>        // write fault and !pte_write(entry)
>        do_wp_page
>          wp_page_copy // this will alloc a new page with valid page struct
>                       // for this pfnmap vma

Here we replace the mapped PFNMAP thingy by a proper anon folio.

> 
> // inject a hwpoison to the first page of this vma

I assume this is an anon folio?

> madvise_inject_error
>    memory_failure
>      hwpoison_user_mappings
>        try_to_unmap_one
>          // mark this pte as invalid (hwpoison)
>          mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
>                  address, range.end);
> 
> // during unmap vma, the first pfn of this pfnmap vma is invalid
> vm_mmap_pgoff
>    do_mmap
>      __do_mmap_mm
>        __mmap_region
>          __do_munmap
>            unmap_region
>              unmap_vmas
>                unmap_single_vma
>                  untrack_pfn
>                    follow_phys // pte is already invalidate, WARN_ON here

unmap_single_vma()->...->zap_pte_range() should do the right thing when 
calling vm_normal_page().

untrack_pfn() is the problematic part.

> 
> CoW with a valid page for pfnmap vma is weird to us. Can we use
> remap_pfn_range for private vma(read only)? Once CoW happens on a pfnmap
> vma during write fault, this page is normal(page flag is valid) for most mm
> subsystems, such as memory failure in thais case and extra should be done to
> handle this special page.
> 
> During unmap, if this vma is pfnmap, unmap shouldn't be done since page
> should not be touched for pfnmap vma.
> 
> But the root problem is that can we insert a valid page for pfnmap vma?
> 
> Any thoughts to solve this warn?

vm_normal_page() documentation explains how that magic is supposed to 
work. vm_normal_page() should be able to correctly identify whether we 
want to look at the struct page for an anon folio that was COWed.


untrack_pfn() indeed does not seem to be well prepared for handling 
MAP_PRIVATE mappings where we end up having anon folios.

I think it will already *completely mess up* simply when unmapping the 
range without the memory failure involved.

See, follow_phys() would get the PFN of the anon folio and then 
untrack_pfn() would do some nonesense with that. Completely broken.

The WARN is just a side-effect of the brokenness.

In follow_phys(), we'd likely have to call vm_normal_page(). If we get a 
page back, we'd likely have to fail follow_phys() instead of returning a 
PFN of an anon folio.

Now, how do we fix untrack_pfn() ? I really don't know. In theory, we 
might no longer have *any* PFNMAP PFN in there after COW'ing everything.

Sounds like MAP_PRIVATE VM_PFNMAP + __HAVE_PFNMAP_TRACKING is some 
broken garbage (sorry). Can we disallow it?

-- 
Cheers,

David / dhildenb