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

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

On 9/5/2022 7:11 PM, David Hildenbrand wrote:
> On 05.09.22 12:24, David Hildenbrand wrote:
>> On 05.09.22 12:16, Baolin Wang wrote:
>>>
>>>
>>> On 9/5/2022 3:59 PM, David Hildenbrand wrote:
>>>> 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.
>>>
>>> IMHO the compiler may optimize the code 'pte_val(*ptep)' to be always
>>> get from a register, then we can get an old value if other thread did
>>> set_pte(). I am not sure how the implicit memory barrier can pervent the
>>> compiler optimization? Please correct me if I missed something.
>>
>> IIUC, an memory barrier always implies a compiler barrier.
>>
> 
> To clarify what I mean, Documentation/atomic_t.txt documents
> 
> NOTE: when the atomic RmW ops are fully ordered, they should also imply 
> a compiler barrier.

Right, I agree. That means the complier can not optimize the order of 
the 'pte_val(*ptep)', however what I am confusing is that the complier 
can still save the value of *ptep into a register or stack instead of 
reloading from memory?

A similar issue in commit d6c1f098f2a7 ("mm/swap_state: fix a data race 
in swapin_nr_pages").

--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -509,10 +509,11 @@ static unsigned long swapin_nr_pages(unsigned long 
offset)
                 return 1;

         hits = atomic_xchg(&swapin_readahead_hits, 0);
-       pages = __swapin_nr_pages(prev_offset, offset, hits, max_pages,
+       pages = __swapin_nr_pages(READ_ONCE(prev_offset), offset, hits,
+                                 max_pages,
                                   atomic_read(&last_readahead_pages));
         if (!hits)
-               prev_offset = offset;
+               WRITE_ONCE(prev_offset, offset);
         atomic_set(&last_readahead_pages, pages);

         return pages;