Re: [linus:master] [mm] efa7df3e3b: kernel_BUG_at_include/linux/page_ref.h

[David Hildenbrand <david@redhat.com>]

On 31.05.24 20:13, Yang Shi wrote:
> On Fri, May 31, 2024 at 11:07 AM Yang Shi <shy828301@gmail.com> wrote:
>>
>> On Fri, May 31, 2024 at 10:46 AM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 31.05.24 18:50, Yang Shi wrote:
>>>> On Fri, May 31, 2024 at 1:24 AM kernel test robot <oliver.sang@intel.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> Hello,
>>>>>
>>>>> kernel test robot noticed "kernel_BUG_at_include/linux/page_ref.h" on:
>>>>>
>>>>> commit: efa7df3e3bb5da8e6abbe37727417f32a37fba47 ("mm: align larger anonymous mappings on THP boundaries")
>>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>>>>>
>>>>> [test failed on linus/master      e0cce98fe279b64f4a7d81b7f5c3a23d80b92fbc]
>>>>> [test failed on linux-next/master 6dc544b66971c7f9909ff038b62149105272d26a]
>>>>>
>>>>> in testcase: trinity
>>>>> version: trinity-x86_64-6a17c218-1_20240527
>>>>> with following parameters:
>>>>>
>>>>>           runtime: 300s
>>>>>           group: group-00
>>>>>           nr_groups: 5
>>>>>
>>>>>
>>>>>
>>>>> compiler: gcc-13
>>>>> test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
>>>>>
>>>>> (please refer to attached dmesg/kmsg for entire log/backtrace)
>>>>>
>>>>>
>>>>> we noticed the issue does not always happen. 34 times out of 50 runs as below.
>>>>> the parent is clean.
>>>>>
>>>>> 1803d0c5ee1a3bbe efa7df3e3bb5da8e6abbe377274
>>>>> ---------------- ---------------------------
>>>>>          fail:runs  %reproduction    fail:runs
>>>>>              |             |             |
>>>>>              :50          68%          34:50    dmesg.Kernel_panic-not_syncing:Fatal_exception
>>>>>              :50          68%          34:50    dmesg.RIP:try_get_folio
>>>>>              :50          68%          34:50    dmesg.invalid_opcode:#[##]
>>>>>              :50          68%          34:50    dmesg.kernel_BUG_at_include/linux/page_ref.h
>>>>>
>>>>>
>>>>>
>>>>> If you fix the issue in a separate patch/commit (i.e. not just a new version of
>>>>> the same patch/commit), kindly add following tags
>>>>> | Reported-by: kernel test robot <oliver.sang@intel.com>
>>>>> | Closes: https://lore.kernel.org/oe-lkp/202405311534.86cd4043-lkp@intel.com
>>>>>
>>>>>
>>>>> [  275.267158][ T4335] ------------[ cut here ]------------
>>>>> [  275.267949][ T4335] kernel BUG at include/linux/page_ref.h:275!
>>>>> [  275.268526][ T4335] invalid opcode: 0000 [#1] KASAN PTI
>>>>> [  275.269001][ T4335] CPU: 0 PID: 4335 Comm: trinity-c3 Not tainted 6.7.0-rc4-00061-gefa7df3e3bb5 #1
>>>>> [  275.269787][ T4335] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
>>>>> [ 275.270679][ T4335] RIP: 0010:try_get_folio (include/linux/page_ref.h:275 (discriminator 3) mm/gup.c:79 (discriminator 3))
>>>>> [ 275.271159][ T4335] Code: c3 cc cc cc cc 44 89 e6 48 89 df e8 e4 54 11 00 eb ae 90 0f 0b 90 31 db eb d5 9c 58 0f 1f 40 00 f6 c4 02 0f 84 46 ff ff ff 90 <0f> 0b 48 c7 c6 a0 54 d2 87 48 89 df e8 a9 e9 ff ff 90 0f 0b be 04
>>>>
>>>> If I read this BUG correctly, it is:
>>>>
>>>> VM_BUG_ON(!in_atomic() && !irqs_disabled());
>>>>
>>>
>>> Yes, that seems to be the one.
>>>
>>>> try_grab_folio() actually assumes it is in an atomic context (irq
>>>> disabled or preempt disabled) for this call path. This is achieved by
>>>> disabling irq in gup fast or calling it in rcu critical section in
>>>> page cache lookup path
>>>
>>> try_grab_folio()->try_get_folio()->folio_ref_try_add_rcu()
>>>
>>> Is called (mm-unstable) from:
>>>
>>> (1) gup_fast function, here IRQs are disable
>>> (2) gup_hugepte(), possibly problematic
>>> (3) memfd_pin_folios(), possibly problematic
>>> (4) __get_user_pages(), likely problematic
>>>
>>> (1) should be fine.
>>>
>>> (2) is possibly problematic on the !fast path. If so, due to commit
>>>       a12083d721d7 ("mm/gup: handle hugepd for follow_page()") ? CCing Peter.
>>>
>>> (3) is possibly wrong. CCing Vivek.
>>>
>>> (4) is what we hit here
>>>
>>>>
>>>> And try_grab_folio() is used when the folio is a large folio. The
>>>
>>>
>>> We come via process_vm_rw()->pin_user_pages_remote()->__get_user_pages()->try_grab_folio()
>>>
>>> That code was added in
>>>
>>> commit 57edfcfd3419b4799353d8cbd6ce49da075cfdbd
>>> Author: Peter Xu <peterx@redhat.com>
>>> Date:   Wed Jun 28 17:53:07 2023 -0400
>>>
>>>       mm/gup: accelerate thp gup even for "pages != NULL"
>>>
>>>       The acceleration of THP was done with ctx.page_mask, however it'll be
>>>       ignored if **pages is non-NULL.
>>>
>>>
>>> Likely the try_grab_folio() in __get_user_pages() is wrong?
>>>
>>> As documented, we already hold a refcount. Likely we should better do a
>>> folio_ref_add() and sanity check the refcount.
>>
>> Yes, a plain folio_ref_add() seems ok for these cases.
>>
>> In addition, the comment of folio_try_get_rcu() says, which is just a
>> wrapper of folio_ref_try_add_rcu():
>>
>> You can also use this function if you're holding a lock that prevents
>> pages being frozen & removed; eg the i_pages lock for the page cache
>> or the mmap_lock or page table lock for page tables.  In this case, it
>> will always succeed, and you could have used a plain folio_get(), but
>> it's sometimes more convenient to have a common function called from
>> both locked and RCU-protected contexts.
>>
>> So IIUC we can use the plain folio_get() at least for
>> process_vm_readv/writev since mmap_lock is held in this path.
>>
>>>
>>>
>>> In essence, I think: try_grab_folio() should only be called from GUP-fast where
>>> IRQs are disabled.
>>
>> Yes, I agree. Just the fast path should need to call try_grab_folio().
> 
> try_grab_folio() also handles FOLL_PIN and FOLL_GET, so we may just
> keep calling it and add a flag to try_grab_folio, just like:
> 
> if flag is true
>      folio_ref_add()
> else
>      try_get_folio()


try_grab_page() is what we use on the GUP-slow path. We'd likely want a 
folio variant of that.

We might want to call that gup_try_grab_folio() and rename the other one 
to gup_fast_try_grab_folio().

Or something like that :)

-- 
Cheers,

David / dhildenb