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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 9/5/2022 6:29 AM, 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().
> 
> That's a rather vague statement, and in fact, the READ_ONCE() should
> be paired with a page table write somewhere else, to make that claim
> more precise.

Agree. We also talked about using READ_ONCE() for *ptep (or using 
ptep_get_lockless()) before in a concurrent scenario of GUP-fast and 
migration [1].

 >>> CPU 0:				CPU 1:
 >>> get_user_pages_fast()
 >>> lockless_pages_from_mm()
 >>> local_irq_save()
 >>> gup_pgd_range()
 >>> gup_p4d_range()
 >>> gup_pud_range()
 >>> gup_pmd_range()
 >>> gup_pte_range()
 >>> pte_t pte = ptep_get_lockless(ptep);
 >>> 				migrate_vma_collect_pmd()
 >>> 				ptep = pte_offset_map_lock(mm, pmdp, addr, &ptl)
 >>> 				ptep_get_and_clear(mm, addr, ptep);
 >>> page = pte_page(pte);
 >>> 				set_pte_at(mm, addr, ptep, swp_pte);
 >>> 				migrate_page_move_mapping()
 >>> head = try_grab_compound_head(page, 1, flags);

[1] 
https://lore.kernel.org/all/7ec1d098-0021-ae82-8d73-dd9c2eb80dab@linux.alibaba.com/

>>   			gup_put_folio(folio, 1, flags);
>>   			goto pte_unmap;
>>   		}
>> @@ -2470,8 +2487,9 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
>>    * get_user_pages_fast_only implementation that can pin pages. Thus it's still
>>    * useful to have gup_huge_pmd even if we can't operate on ptes.
>>    */
>> -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)
>>   {
>>   	return 0;
>>   }
>> @@ -2791,7 +2809,7 @@ static int gup_pmd_range(pud_t *pudp, pud_t pud, unsigned long addr, unsigned lo
>>   			if (!gup_huge_pd(__hugepd(pmd_val(pmd)), addr,
>>   					 PMD_SHIFT, next, flags, pages, nr))
>>   				return 0;
>> -		} else if (!gup_pte_range(pmd, addr, next, flags, pages, nr))
>> +		} else if (!gup_pte_range(pmd, pmdp, addr, next, flags, pages, nr))
>>   			return 0;
>>   	} while (pmdp++, addr = next, addr != end);
>>   
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 2d74cf01f694..518b49095db3 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -1049,10 +1049,12 @@ static int collapse_huge_page(struct mm_struct *mm, unsigned long address,
>>   
>>   	pmd_ptl = pmd_lock(mm, pmd); /* probably unnecessary */
>>   	/*
>> -	 * 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.
> 
> 
> thanks,
>