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

[Yang Shi]

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?

>
> This chunk of code was (mostly) originally added in 2008 in commit
> e286781d5f2e ("mm: speculative page references").
>
> In short, I'm wondering whether something like below would make sense and
> easier:
>
> ===8<===
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 1acf5bac7f50..c89a67d239d1 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -258,26 +258,9 @@ static inline bool folio_try_get(struct folio *folio)
>         return folio_ref_add_unless(folio, 1, 0);
>  }
>
> -static inline bool folio_ref_try_add_rcu(struct folio *folio, int count)
> -{
> -#ifdef CONFIG_TINY_RCU
> -       /*
> -        * The caller guarantees the folio will not be freed from interrupt
> -        * context, so (on !SMP) we only need preemption to be disabled
> -        * and TINY_RCU does that for us.
> -        */
> -# ifdef CONFIG_PREEMPT_COUNT
> -       VM_BUG_ON(!in_atomic() && !irqs_disabled());
> -# endif
> -       VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio);
> -       folio_ref_add(folio, count);
> -#else
> -       if (unlikely(!folio_ref_add_unless(folio, count, 0))) {
> -               /* Either the folio has been freed, or will be freed. */
> -               return false;
> -       }
> -#endif
> -       return true;
> +static inline bool folio_ref_try_add(struct folio *folio, int count)
> +{
> +       return folio_ref_add_unless(folio, count, 0);
>  }
>
>  /**
> @@ -305,7 +288,7 @@ static inline bool folio_ref_try_add_rcu(struct folio *folio, int count)
>   */
>  static inline bool folio_try_get_rcu(struct folio *folio)
>  {
> -       return folio_ref_try_add_rcu(folio, 1);
> +       return folio_ref_try_add(folio, 1);
>  }
>
>  static inline int page_ref_freeze(struct page *page, int count)
> diff --git a/mm/gup.c b/mm/gup.c
> index e17466fd62bb..17f89e8d31f1 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -78,7 +78,7 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
>         folio = page_folio(page);
>         if (WARN_ON_ONCE(folio_ref_count(folio) < 0))
>                 return NULL;
> -       if (unlikely(!folio_ref_try_add_rcu(folio, refs)))
> +       if (unlikely(!folio_ref_try_add(folio, refs)))
>                 return NULL;
>
>         /*
> ===8<===
>
> So instead of adding new code, we fix it by removing some.  There might be
> some implication on TINY_RCU / UP config on using the atomic_add_unless()
> to replace one atomic_add(), but I'm not sure whether that's a major issue.
>
> The benefit is try_get_folio() then don't need a renaming then, because the
> rcu implication just went away.
>
> Thanks,
>
> --
> Peter Xu
>