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

[mawupeng]

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.

It do seems wired for private mappings, What is the good reason?

> 
> just a device fd with device-specify mmap which use remap_pfn_range to assign memory.
> 
>>>
>>>> // 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.
> 
> My problem is can wen replace a pfn with fully functioned page for pfnmap vma? This is not MIXEDMAP vma.
> 
>>>
>>>>
>>>> // inject a hwpoison to the first page of this vma
>>>
>>> I assume this is an anon folio?
> 
> Yes.
> 
>>>
>>>> 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);
> 
> If we can replace the mapped PFNMAP thingy by a proper anon folio, we need to make memory_failure to handle
> pfnmap vma properly since pfnmap vma shoule not touch its struct page?
> 
> Current this page have a valid mapping and can be unmap.
> 
> Maybe there is something wrong with my understanding of CoW on a private pfnmap vma.
> 
>>>>
>>>> // 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.
> 
> For pfnmap vma, it don't have a valid page for all pfns, so unmap is not expected. In this case, it just
> check wheather the first address have a valid pte or not which seems reasonable to me.
> 
>>>
>>>>
>>>> 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.
> 
> vm_normal_page() can find out a CoW mapping but 
> 
>>>
>>>
>>> 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?
>>
>> Staring at track_pfn_copy(), it's maybe similarly broken?
>>
>> I think we want to do:
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 098356b8805ae..da5d1e37c5534 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -6050,6 +6050,10 @@ int follow_phys(struct vm_area_struct *vma,
>>                 goto out;
>>         pte = ptep_get(ptep);
>>  
>> +       /* Never return addresses of COW'ed anon folios. */
>> +       if (vm_normal_page(vma, address, pte))
>> +               goto unlock;
>> +
>>         if ((flags & FOLL_WRITE) && !pte_write(pte))
>>                 goto unlock;
>>  
>>
>> And then, just disallow it with PAT involved:
>>
>> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
>> index 0904d7e8e1260..e4d2b2e8c0281 100644
>> --- a/arch/x86/mm/pat/memtype.c
>> +++ b/arch/x86/mm/pat/memtype.c
>> @@ -997,6 +997,15 @@ int track_pfn_remap(struct vm_area_struct *vma, pgprot_t *prot,
>>                                 && size == (vma->vm_end - vma->vm_start))) {
>>                 int ret;
>>  
>> +               /*
>> +                * untrack_pfn() and friends cannot handl regions that suddenly
>> +                * contain anon folios after COW. In particular, follow_phys()
>> +                * will fail when we have an anon folio at the beginning og the
>> +                * VMA.
>> +                */
>> +               if (vma && is_cow_mapping(vma->vm_flags))
>> +                       return -EINVAL;
> 
> In this case, anyone use remap_pfn_range can not be cow_maaping which means if VM_MAYWRITE exists, VM_SHARED is
> needed for this vma.
> 
> This can solve this CoW on private vma problem.
> 
>> +
>>                 ret = reserve_pfn_range(paddr, size, prot, 0);
>>                 if (ret == 0 && vma)
>>                         vm_flags_set(vma, VM_PAT);
>>
>>
>> I'm afraid that will break something. But well, it's already semi-broken.
>>
>> As long as VM_PAT is not involved, it should work as expected.
>>
>> In an ideal world, we'd get rid of follow_phys() completely and just
>> derive that information from the VMA?
>>