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

Yang Shi
2024-05-31T18:07:36Z

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().

>
> (2), (3) and (4) are possible offenders of that.
>
>
> Or am I getting it all wrong? :)
>
> --
> Cheers,
>
> David / dhildenb
>