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

[David Hildenbrand]

On 01.06.24 02:59, Yang Shi wrote:
> On Fri, May 31, 2024 at 5:01 PM Yang Shi <shy828301@xxxxxxxxx> wrote:
> > On Fri, May 31, 2024 at 4:25 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> > > On Fri, May 31, 2024 at 07:46:41PM +0200, David Hildenbrand wrote:
> > > > 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
> > >
> > > I guess it was overlooked because try_grab_folio() didn't have any comment
> > > or implication on RCU or IRQ internal helpers being used, hence a bit
> > > confusing.  E.g. it has different context requirement on try_grab_page(),
> > > even though they look like sister functions.  It might be helpful to have a
> > > better name, something like try_grab_folio_rcu() in this case.
> > >
> > > Btw, none of above cases (2-4) have real bug, but we're looking at some way
> > > to avoid triggering the sanity check, am I right?  I hope besides the host
> > > splash I didn't overlook any other side effect this issue would cause, and
> > > the splash IIUC should so far be benign, as either gup slow (2,4) or the
> > > newly added memfd_pin_folios() (3) look like to have the refcount stablized
> > > anyway.
> > >
> > > Yang's patch in the other email looks sane to me, just that then we'll add
> > > quite some code just to avoid this sanity check in paths 2-4 which seems
> > > like an slight overkill.
> > >
> > > One thing I'm thinking is whether folio_ref_try_add_rcu() can get rid of
> > > its RCU limitation. It boils down to whether we can use atomic_add_unless()
> > > on TINY_RCU / UP setup too?  I mean, we do plenty of similar things
> > > (get_page_unless_zero, etc.) in generic code and I don't understand why
> > > here we need to treat folio_ref_try_add_rcu() specially.
> > >
> > > IOW, the assertions here we added:
> > >
> > >          VM_BUG_ON(!in_atomic() && !irqs_disabled());
> > >
> > > Is because we need atomicity of below sequences:
> > >
> > >          VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio);
> > >          folio_ref_add(folio, count);
> > >
> > > But atomic ops avoids it.
> >
> > Yeah, I didn't think of why atomic can't do it either. But is it
> > written in this way because we want to catch the refcount == 0 case
> > since it means a severe bug? Did we miss something?
>
> Thought more about it and disassembled the code. IIUC, this is an
> optimization for non-SMP kernel. When in rcu critical section or irq
> is disabled, we just need an atomic add instruction.
> folio_ref_add_unless() would yield more instructions, including branch
> instruction. But I'm wondering how useful it would be nowadays. Is it
> really worth the complexity? AFAIK, for example, ARM64 has not
> supported non-SMP kernel for years.
>
> My patch actually replaced all folio_ref_add_unless() to
> folio_ref_add() for slow paths, so it is supposed to run faster, but
> we are already in slow path, it may be not measurable at all. So
> having more simple and readable code may outweigh the potential slight
> performance gain in this case?

Yes, we don't want to use atomic RMW that return values where we can use 
atomic RMW that don't return values. The former is slower and implies a 
memory barrier, that can be optimized out on some arcitectures (arm64 IIRC)

We should clean that up here, and make it clearer that the old function 
is only for grabbing a folio if it can be freed concurrently -- GUP-fast.

--
Cheers,

David / dhildenb