Re: [PATCH v11 10/26] mm: protect VMA modifications using VMA sequence count

[Vinayak Menon <vinmenon@codeaurora.org>]

On 11/5/2018 11:52 PM, Laurent Dufour wrote:
> Le 05/11/2018 à 08:04, vinayak menon a écrit :
>> Hi Laurent,
>>
>> On Thu, May 17, 2018 at 4:37 PM Laurent Dufour
>> <ldufour@linux.vnet.ibm.com> wrote:
>>>
>>> The VMA sequence count has been introduced to allow fast detection of
>>> VMA modification when running a page fault handler without holding
>>> the mmap_sem.
>>>
>>> This patch provides protection against the VMA modification done in :
>>>          - madvise()
>>>          - mpol_rebind_policy()
>>>          - vma_replace_policy()
>>>          - change_prot_numa()
>>>          - mlock(), munlock()
>>>          - mprotect()
>>>          - mmap_region()
>>>          - collapse_huge_page()
>>>          - userfaultd registering services
>>>
>>> In addition, VMA fields which will be read during the speculative fault
>>> path needs to be written using WRITE_ONCE to prevent write to be split
>>> and intermediate values to be pushed to other CPUs.
>>>
>>> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>>> ---
>>>   fs/proc/task_mmu.c |  5 ++++-
>>>   fs/userfaultfd.c   | 17 +++++++++++++----
>>>   mm/khugepaged.c    |  3 +++
>>>   mm/madvise.c       |  6 +++++-
>>>   mm/mempolicy.c     | 51 ++++++++++++++++++++++++++++++++++-----------------
>>>   mm/mlock.c         | 13 ++++++++-----
>>>   mm/mmap.c          | 22 +++++++++++++---------
>>>   mm/mprotect.c      |  4 +++-
>>>   mm/swap_state.c    |  8 ++++++--
>>>   9 files changed, 89 insertions(+), 40 deletions(-)
>>>
>>>   struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>>>                                  struct vm_fault *vmf)
>>> @@ -665,9 +669,9 @@ static inline void swap_ra_clamp_pfn(struct vm_area_struct *vma,
>>>                                       unsigned long *start,
>>>                                       unsigned long *end)
>>>   {
>>> -       *start = max3(lpfn, PFN_DOWN(vma->vm_start),
>>> +       *start = max3(lpfn, PFN_DOWN(READ_ONCE(vma->vm_start)),
>>>                        PFN_DOWN(faddr & PMD_MASK));
>>> -       *end = min3(rpfn, PFN_DOWN(vma->vm_end),
>>> +       *end = min3(rpfn, PFN_DOWN(READ_ONCE(vma->vm_end)),
>>>                      PFN_DOWN((faddr & PMD_MASK) + PMD_SIZE));
>>>   }
>>>
>>> -- 
>>> 2.7.4
>>>
>>
>> I have got a crash on 4.14 kernel with speculative page faults enabled
>> and here is my analysis of the problem.
>> The issue was reported only once.
>
> Hi Vinayak,
>
> Thanks for reporting this.
>
>>
>> [23409.303395]  el1_da+0x24/0x84
>> [23409.303400]  __radix_tree_lookup+0x8/0x90
>> [23409.303407]  find_get_entry+0x64/0x14c
>> [23409.303410]  pagecache_get_page+0x5c/0x27c
>> [23409.303416]  __read_swap_cache_async+0x80/0x260
>> [23409.303420]  swap_vma_readahead+0x264/0x37c
>> [23409.303423]  swapin_readahead+0x5c/0x6c
>> [23409.303428]  do_swap_page+0x128/0x6e4
>> [23409.303431]  handle_pte_fault+0x230/0xca4
>> [23409.303435]  __handle_speculative_fault+0x57c/0x7c8
>> [23409.303438]  do_page_fault+0x228/0x3e8
>> [23409.303442]  do_translation_fault+0x50/0x6c
>> [23409.303445]  do_mem_abort+0x5c/0xe0
>> [23409.303447]  el0_da+0x20/0x24
>>
>> Process A accesses address ADDR (part of VMA A) and that results in a
>> translation fault.
>> Kernel enters __handle_speculative_fault to fix the fault.
>> Process A enters do_swap_page->swapin_readahead->swap_vma_readahead
>> from speculative path.
>> During this time, another process B which shares the same mm, does a
>> mprotect from another CPU which follows
>> mprotect_fixup->__split_vma, and it splits VMA A into VMAs A and B.
>> After the split, ADDR falls into VMA B, but process A is still using
>> VMA A.
>> Now ADDR is greater than VMA_A->vm_start and VMA_A->vm_end.
>> swap_vma_readahead->swap_ra_info uses start and end of vma to
>> calculate ptes and nr_pte, which goes wrong due to this and finally
>> resulting in wrong "entry" passed to
>> swap_vma_readahead->__read_swap_cache_async, and in turn causing
>> invalid swapper_space
>> being passed to __read_swap_cache_async->find_get_page, causing an abort.
>>
>> The fix I have tried is to cache vm_start and vm_end also in vmf and
>> use it in swap_ra_clamp_pfn. Let me know your thoughts on this. I can
>> send
>> the patch I am a using if you feel that is the right thing to do.
>
> I think the best would be to don't do swap readahead during the speculatvive page fault. If the page is found in the swap cache, that's fine, but otherwise, we should f    allback to the regular page fault.
>
> The attached -untested- patch is doing this, if you want to give it a try. I'll review that for the next series.
>

Thanks Laurent. I and going to try this patch.

With this patch, since all non-SWP_SYNCHRONOUS_IO swapins result in non-speculative fault
and a retry, wouldn't this have an impact on some perf numbers ? If so, would caching start
and end be a better option ?

Also, would it make sense to move the FAULT_FLAG_SPECULATIVE check inside swapin_readahead,
in a way that  swap_cluster_readahead can take the speculative path ? swap_cluster_readahead
doesn't seem to use vma values.

Thanks,
Vinayak