Re: [PATCH] mm: fix a data race in put_page()

[David Hildenbrand]

On 06.02.20 15:55, Jan Kara wrote:
> On Thu 06-02-20 14:33:22, David Hildenbrand wrote:
>> On 06.02.20 14:17, Qian Cai wrote:
>>> page->flags could be accessed concurrently as noticied by KCSAN,
>>>
>>>  BUG: KCSAN: data-race in page_cpupid_xchg_last / put_page
>>>
>>>  write (marked) to 0xfffffc0d48ec1a00 of 8 bytes by task 91442 on cpu 3:
>>>   page_cpupid_xchg_last+0x51/0x80
>>>   page_cpupid_xchg_last at mm/mmzone.c:109 (discriminator 11)
>>>   wp_page_reuse+0x3e/0xc0
>>>   wp_page_reuse at mm/memory.c:2453
>>>   do_wp_page+0x472/0x7b0
>>>   do_wp_page at mm/memory.c:2798
>>>   __handle_mm_fault+0xcb0/0xd00
>>>   handle_pte_fault at mm/memory.c:4049
>>>   (inlined by) __handle_mm_fault at mm/memory.c:4163
>>>   handle_mm_fault+0xfc/0x2f0
>>>   handle_mm_fault at mm/memory.c:4200
>>>   do_page_fault+0x263/0x6f9
>>>   do_user_addr_fault at arch/x86/mm/fault.c:1465
>>>   (inlined by) do_page_fault at arch/x86/mm/fault.c:1539
>>>   page_fault+0x34/0x40
>>>
>>>  read to 0xfffffc0d48ec1a00 of 8 bytes by task 94817 on cpu 69:
>>>   put_page+0x15a/0x1f0
>>>   page_zonenum at include/linux/mm.h:923
>>>   (inlined by) is_zone_device_page at include/linux/mm.h:929
>>>   (inlined by) page_is_devmap_managed at include/linux/mm.h:948
>>>   (inlined by) put_page at include/linux/mm.h:1023
>>>   wp_page_copy+0x571/0x930
>>>   wp_page_copy at mm/memory.c:2615
>>>   do_wp_page+0x107/0x7b0
>>>   __handle_mm_fault+0xcb0/0xd00
>>>   handle_mm_fault+0xfc/0x2f0
>>>   do_page_fault+0x263/0x6f9
>>>   page_fault+0x34/0x40
>>>
>>>  Reported by Kernel Concurrency Sanitizer on:
>>>  CPU: 69 PID: 94817 Comm: systemd-udevd Tainted: G        W  O L 5.5.0-next-20200204+ #6
>>>  Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
>>>
>>> Both the read and write are done only with the non-exclusive mmap_sem
>>> held. Since the read will check for specific bits (up to three bits for
>>> now) in the flag, load tearing could in theory trigger a logic bug.
>>>
>>> To fix it, it could introduce put_page_lockless() in those places but
>>> that could be an overkill, and difficult to use. Thus, just add
>>> READ_ONCE() for the read in page_zonenum() for now where it should not
>>> affect the performance and correctness with a small trade-off that
>>> compilers might generate less efficient optimization in some places.
>>>
>>> Signed-off-by: Qian Cai <cai@xxxxxx>
>>> ---
>>>  include/linux/mm.h | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>> index 52269e56c514..f8529aa971c0 100644
>>> --- a/include/linux/mm.h
>>> +++ b/include/linux/mm.h
>>> @@ -920,7 +920,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>>>  
>>>  static inline enum zone_type page_zonenum(const struct page *page)
>>>  {
>>> -	return (page->flags >> ZONES_PGSHIFT) & ZONES_MASK;
>>> +	return (READ_ONCE(page->flags) >> ZONES_PGSHIFT) & ZONES_MASK;
>>
>> I can understand why other bits/flags might change, but not the zone
>> number? Nobody should be changing that without heavy locking (out of
>> memory hot(un)plug code). Or am I missing something? Can load tearing
>> actually produce an issue if these 3 bits will never change?
> 
> I don't think the problem is real. The question is how to make KCSAN happy
> in a way that doesn't silence other possibly useful things it can find and
> also which makes it most obvious to the reader what's going on... IMHO
> using READ_ONCE() fulfills these targets nicely - it is free
> performance-wise in this case, it silences the checker without impacting
> other races on page->flags, its kind of obvious we don't want the load torn
> in this case so it makes sense to the reader (although a comment may be
> nice).

Yes, I can understand that - but reading the other comments, I think I
am not the only one who doesn't like to see the code getting uglyfied
(sorry, but that's what it is) and harder to read just to make some code
checker happy.

Having that said, I don't really care. But I do think that expressing
"this is not possible but only to make $TOOL happy" in the commit
message is worth having. The patch description should be reworked.

-- 
Thanks,

David / dhildenb