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

Yang Shi
2024-05-31T18:13:35Z

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()

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