linux-kernel - Re: [RFC PATCH RESEND 04/28] mm: move mmap

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Date:   Thu, 1 Sep 2022 16:21:47 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     Kent Overstreet <kent.overstreet@...ux.dev>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "michel@...pinasse.org" <michel@...pinasse.org>,
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Subject: Re: [RFC PATCH RESEND 04/28] mm: move mmap_lock assert function definitions

On Thu, Sep 1, 2022 at 1:51 PM Liam Howlett <liam.howlett@...cle.com> wrote:
>
> * Kent Overstreet <kent.overstreet@...ux.dev> [220901 16:24]:
> > On Thu, Sep 01, 2022 at 10:34:52AM -0700, Suren Baghdasaryan wrote:
> > > Move mmap_lock assert function definitions up so that they can be used
> > > by other mmap_lock routines.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> > > ---
> > >  include/linux/mmap_lock.h | 24 ++++++++++++------------
> > >  1 file changed, 12 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> > > index 96e113e23d04..e49ba91bb1f0 100644
> > > --- a/include/linux/mmap_lock.h
> > > +++ b/include/linux/mmap_lock.h
> > > @@ -60,6 +60,18 @@ static inline void __mmap_lock_trace_released(struct mm_struct *mm, bool write)
> > >
> > >  #endif /* CONFIG_TRACING */
> > >
> > > +static inline void mmap_assert_locked(struct mm_struct *mm)
> > > +{
> > > +   lockdep_assert_held(&mm->mmap_lock);
> > > +   VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
> >
> > These look redundant to me - maybe there's a reason the VM developers want both,
> > but I would drop the VM_BUG_ON() and just keep the lockdep_assert_held(), since
> > that's the standard way to write that assertion.
>
> I think this is because the VM_BUG_ON_MM() will give you a lot more
> information and BUG_ON().
>
> lockdep_assert_held() does not return a value and is a WARN_ON().
>
> So they are partially redundant.

Yeah and I do not intend to change the existing functionality in this
patchset. If needed we can post a separate patch removing the
redundancy but from my experience debugging this code, VM_BUG_ON_MM
reports were very useful.

>
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