Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

[David Hildenbrand <david@redhat.com>]

On 05.09.22 00:29, John Hubbard wrote:
> On 9/1/22 15:27, Yang Shi wrote:
>> Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm:
>> introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer
>> sufficient to handle concurrent GUP-fast in all cases, it only handles
>> traditional IPI-based GUP-fast correctly.  On architectures that send
>> an IPI broadcast on TLB flush, it works as expected.  But on the
>> architectures that do not use IPI to broadcast TLB flush, it may have
>> the below race:
>>
>>     CPU A                                          CPU B
>> THP collapse                                     fast GUP
>>                                                gup_pmd_range() <-- see valid pmd
>>                                                    gup_pte_range() <-- work on pte
>> pmdp_collapse_flush() <-- clear pmd and flush
>> __collapse_huge_page_isolate()
>>      check page pinned <-- before GUP bump refcount
>>                                                        pin the page
>>                                                        check PTE <-- no change
>> __collapse_huge_page_copy()
>>      copy data to huge page
>>      ptep_clear()
>> install huge pmd for the huge page
>>                                                        return the stale page
>> discard the stale page
> 
> Hi Yang,
> 
> Thanks for taking the trouble to write down these notes. I always
> forget which race we are dealing with, and this is a great help. :)
> 
> More...
> 
>>
>> The race could be fixed by checking whether PMD is changed or not after
>> taking the page pin in fast GUP, just like what it does for PTE.  If the
>> PMD is changed it means there may be parallel THP collapse, so GUP
>> should back off.
>>
>> Also update the stale comment about serializing against fast GUP in
>> khugepaged.
>>
>> Fixes: 2667f50e8b81 ("mm: introduce a general RCU get_user_pages_fast()")
>> Signed-off-by: Yang Shi <shy828301@gmail.com>
>> ---
>>   mm/gup.c        | 30 ++++++++++++++++++++++++------
>>   mm/khugepaged.c | 10 ++++++----
>>   2 files changed, 30 insertions(+), 10 deletions(-)
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index f3fc1f08d90c..4365b2811269 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -2380,8 +2380,9 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
>>   }
>>   
>>   #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
>> -static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>> -			 unsigned int flags, struct page **pages, int *nr)
>> +static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
>> +			 unsigned long end, unsigned int flags,
>> +			 struct page **pages, int *nr)
>>   {
>>   	struct dev_pagemap *pgmap = NULL;
>>   	int nr_start = *nr, ret = 0;
>> @@ -2423,7 +2424,23 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>>   			goto pte_unmap;
>>   		}
>>   
>> -		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
>> +		/*
>> +		 * THP collapse conceptually does:
>> +		 *   1. Clear and flush PMD
>> +		 *   2. Check the base page refcount
>> +		 *   3. Copy data to huge page
>> +		 *   4. Clear PTE
>> +		 *   5. Discard the base page
>> +		 *
>> +		 * So fast GUP may race with THP collapse then pin and
>> +		 * return an old page since TLB flush is no longer sufficient
>> +		 * to serialize against fast GUP.
>> +		 *
>> +		 * Check PMD, if it is changed just back off since it
>> +		 * means there may be parallel THP collapse.
>> +		 */
> 
> As I mentioned in the other thread, it would be a nice touch to move
> such discussion into the comment header.
> 
>> +		if (unlikely(pmd_val(pmd) != pmd_val(*pmdp)) ||
>> +		    unlikely(pte_val(pte) != pte_val(*ptep))) {
> 
> 
> That should be READ_ONCE() for the *pmdp and *ptep reads. Because this
> whole lockless house of cards may fall apart if we try reading the
> page table values without READ_ONCE().

I came to the conclusion that the implicit memory barrier when grabbing 
a reference on the page is sufficient such that we don't need READ_ONCE 
here.

If we still intend to change that code, we should fixup all GUP-fast 
functions in a similar way. But again, I don't think we need a change here.


>> -	 * After this gup_fast can't run anymore. This also removes
>> -	 * any huge TLB entry from the CPU so we won't allow
>> -	 * huge and small TLB entries for the same virtual address
>> -	 * to avoid the risk of CPU bugs in that area.
>> +	 * This removes any huge TLB entry from the CPU so we won't allow
>> +	 * huge and small TLB entries for the same virtual address to
>> +	 * avoid the risk of CPU bugs in that area.
>> +	 *
>> +	 * Parallel fast GUP is fine since fast GUP will back off when
>> +	 * it detects PMD is changed.
>>   	 */
>>   	_pmd = pmdp_collapse_flush(vma, address, pmd);
> 
> To follow up on David Hildenbrand's note about this in the nearby thread...
> I'm also not sure if pmdp_collapse_flush() implies a memory barrier on
> all arches. It definitely does do an atomic op with a return value on x86,
> but that's just one arch.
> 

I think a ptep/pmdp clear + TLB flush really has to imply a memory 
barrier, otherwise TLB flushing code might easily mess up with 
surrounding code. But we should better double-check.

s390x executes an IDTE instruction, which performs serialization (-> 
memory barrier). arm64 seems to use DSB instructions to enforce memory 
ordering.

-- 
Thanks,

David / dhildenb