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

[David Hildenbrand <david@redhat.com>]

On 05.09.22 16:35, Baolin Wang wrote:
> 
> 
> 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?

After the memory+compiler barrier, the value has to be reloaded. 
Documentation/memory-barriers.txt documents under "COMPILER BARRIERS":

"READ_ONCE() and WRITE_ONCE() can be thought of as weak forms of 
barrier() that affect only the specific accesses flagged by the 
READ_ONCE() or WRITE_ONCE()."

Consequently, if there already is a compile barrier, additional 
READ_ONCE/WRITE_ONCE isn't required.

> 
> 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;
> 

IIUC the difference here is that there is not other implicit 
memory+compile barrier in between.

-- 
Thanks,

David / dhildenb