Re: Avoiding allocation of unused shmem page

David Hildenbrand
2022-10-21T16:19:28Z

On 21.10.22 18:01, Peter Xu wrote:
> On Fri, Oct 21, 2022 at 05:17:08PM +0200, David Hildenbrand wrote:
>> On 21.10.22 17:08, Peter Xu wrote:
>>> On Fri, Oct 21, 2022 at 04:45:27PM +0200, David Hildenbrand wrote:
>>>> On 21.10.22 16:28, Peter Xu wrote:
>>>>> On Fri, Oct 21, 2022 at 04:10:41PM +0200, David Hildenbrand wrote:
>>>>>> On 21.10.22 16:01, Peter Xu wrote:
>>>>>>> On Fri, Oct 21, 2022 at 09:23:00AM +0200, David Hildenbrand wrote:
>>>>>>>> On 20.10.22 23:10, Peter Xu wrote:
>>>>>>>>> On Thu, Oct 20, 2022 at 09:14:09PM +0100, Matthew Wilcox wrote:
>>>>>>>>>> In yesterday's call, David brought up the case where we fallocate a file
>>>>>>>>>> in shmem, call mmap(MAP_PRIVATE) and then store to a page which is over
>>>>>>>>>> a hole. That currently causes shmem to allocate a page, zero-fill it,
>>>>>>>>>> then COW it, resulting in two pages being allocated when only the
>>>>>>>>>> COW page really needs to be allocated.
>>>>>>>>>>
>>>>>>>>>> The path we currently take through the MM when we take the page fault
>>>>>>>>>> looks like this (correct me if I'm wrong ...):
>>>>>>>>>>
>>>>>>>>>> handle_mm_fault()
>>>>>>>>>> __handle_mm_fault()
>>>>>>>>>> handle_pte_fault()
>>>>>>>>>> do_fault()
>>>>>>>>>> do_cow_fault()
>>>>>>>>>> __do_fault()
>>>>>>>>>> vm_ops->fault()
>>>>>>>>>>
>>>>>>>>>> ... which is where we come into shmem_fault(). Apart from the
>>>>>>>>>> horrendous hole-punch handling case, shmem_fault() is quite simple:
>>>>>>>>>>
>>>>>>>>>> err = shmem_get_folio_gfp(inode, vmf->pgoff, &folio, SGP_CACHE,
>>>>>>>>>> gfp, vma, vmf, &ret);
>>>>>>>>>> if (err)
>>>>>>>>>> return vmf_error(err);
>>>>>>>>>> vmf->page = folio_file_page(folio, vmf->pgoff);
>>>>>>>>>> return ret;
>>>>>>>>>>
>>>>>>>>>> What we could do here is detect this case. Something like:
>>>>>>>>>>
>>>>>>>>>> enum sgp_type sgp = SGP_CACHE;
>>>>>>>>>>
>>>>>>>>>> if ((vmf->flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED))
>>>>>>>>>> sgp = SGP_READ;
>>>>>>>>>
>>>>>>>>> Yes this will start to save the space, but just to mention this may start
>>>>>>>>> to break anything that will still depend on the pagecache to work. E.g.,
>>>>>>>>> it'll change behavior if the vma is registered with uffd missing mode;
>>>>>>>>> we'll start to lose MISSING events for these private mappings. Not sure
>>>>>>>>> whether there're other side effects.
>>>>>>>>
>>>>>>>> I don't follow, can you elaborate?
>>>>>>>>
>>>>>>>> hugetlb doesn't perform this kind of unnecessary allocation and should be fine in regards to uffd. Why should it matter here and how exactly would a problematic sequence look like?
>>>>>>>
>>>>>>> Hugetlb is special because hugetlb detects pte first and relies on pte at
>>>>>>> least for uffd. shmem is not.
>>>>>>>
>>>>>>> Feel free to also reference the recent fix which relies on the stable
>>>>>>> hugetlb pte with commit 2ea7ff1e39cbe375.
>>>>>>
>>>>>> Sorry to be dense here, but I don't follow how that relates.
>>>>>>
>>>>>> Assume we have a MAP_PRIVATE shmem mapping and someone registers uffd
>>>>>> missing events on that mapping.
>>>>>>
>>>>>> Assume we get a page fault on a hole. We detect no page is mapped and check
>>>>>> if the page cache has a page mapped -- which is also not the case, because
>>>>>> there is a hole.
>>>>>>
>>>>>> So we notify uffd.
>>>>>>
>>>>>> Uffd will place a page. It should *not* touch the page cache and only insert
>>>>>> that page into the page table -- otherwise we'd be violating MAP_PRIVATE
>>>>>> semantics.
>>>>>
>>>>> That's actually exactly what we do right now... we insert into page cache
>>>>> for the shmem. See shmem_mfill_atomic_pte().
>>>>>
>>>>> Why it violates MAP_PRIVATE? Private pages only guarantee the exclusive
>>>>> ownership of pages, I don't see why it should restrict uffd behavior. Uffd
>>>>> missing mode (afaiu) is defined to resolve page cache missings in this
>>>>> case. Hugetlb is special but not shmem IMO comparing to most of the rest
>>>>> of the file systems.
>>>>
>>>> If a write (or uffd placement) via a MAP_PRIVATE mapping results in other
>>>> shared/private mappings from observing these modifications, you have a clear
>>>> violation of MAP_PRIVATE semantics.
>>>
>>> I think I understand what you meant, but just to mention again that I think
>>> it's a matter of how we defined the uffd missing semantics when shmem
>>> missing mode was introduced years ago. It does not need to be the same
>>> semantic as writting directly to a private mapping.
>>>
>>
>> I think uffd does exactly the right thing in mfill_atomic_pte:
>>
>> /*
>> * The normal page fault path for a shmem will invoke the
>> * fault, fill the hole in the file and COW it right away. The
>> * result generates plain anonymous memory. So when we are
>> * asked to fill an hole in a MAP_PRIVATE shmem mapping, we'll
>> * generate anonymous memory directly without actually filling
>> * the hole. For the MAP_PRIVATE case the robustness check
>> * only happens in the pagetable (to verify it's still none)
>> * and not in the radix tree.
>> */
>> if (!(dst_vma->vm_flags & VM_SHARED)) {
>> if (mode == MCOPY_ATOMIC_NORMAL)
>> err = mcopy_atomic_pte(dst_mm, dst_pmd, dst_vma,
>> dst_addr, src_addr, page,
>> wp_copy);
>> else
>> err = mfill_zeropage_pte(dst_mm, dst_pmd,
>> dst_vma, dst_addr);
>> } else {
>> err = shmem_mfill_atomic_pte(dst_mm, dst_pmd, dst_vma,
>> dst_addr, src_addr,
>> mode != MCOPY_ATOMIC_NORMAL,
>> wp_copy, page);
>> }
>>
>> Unless we have a writable shared mapping, we end up not touching the pagecache.
>>
>> After what I understand from your last message (maybe I understood it wrong),
>> I tried exploiting uffd behavior by writing into a hole of a file without
>> write permissions using uffd. I failed because it does the right thing ;)
>
> Very interesting to learn this, thanks for the pointer, David. :)
> Definitely helpful to me on knowing better on the vma security model.
>
> Though note that it'll be a different topic if we go back to the original
> problem we're discussing - the fake-read approach of shmem will still keep
> the hole in file which will still change the behavior of missing messages
> from generating.
>
> Said that, I don't really know whether there can be a real impact on any
> uffd users, or anything else that similarly access the file cache.

One odd behavior I could think of is if one would have someone a process
A that uses uffd on a MAP_SHARED shmem and someone other process B
(e.g., with read-only permissions) have a MAP_PRIVATE mapping on the
same file.

A read (or a write) from process B to the private mapping would result
in process A not receiving uffd events.

Of course, the same would happen if you have multiple MAP_SHARED
mappings as well ... but it feels a bit weird being able to do that
without write permissions to the file.

--
Thanks,

David / dhildenb