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

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Date:   Fri, 2 Sep 2022 08:23:46 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Kent Overstreet <kent.overstreet@...ux.dev>
Cc:     Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org,
        michel@...pinasse.org, jglisse@...gle.com, mhocko@...e.com,
        vbabka@...e.cz, hannes@...xchg.org, mgorman@...e.de,
        dave@...olabs.net, willy@...radead.org, liam.howlett@...cle.com,
        peterz@...radead.org, ldufour@...ux.ibm.com,
        laurent.dufour@...ibm.com, paulmck@...nel.org, luto@...nel.org,
        songliubraving@...com, peterx@...hat.com, david@...hat.com,
        dhowells@...hat.com, hughd@...gle.com, rientjes@...gle.com,
        axelrasmussen@...gle.com, joelaf@...gle.com, minchan@...gle.com,
        kernel-team@...roid.com, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH RESEND 04/28] mm: move mmap_lock assert function
 definitions

On 2022-09-01 16:24:09 [-0400], Kent Overstreet wrote:
> > --- 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.

Exactly. rwsem_is_locked() returns true only if the lock is "locked" not
necessary by the caller. lockdep_assert_held() checks that the lock is
locked by the caller - this is the important part.

Sebastian