linux-kernel - Re: [PATCH 1/8] mmap locking API: initial implementation as rwsem wrappers

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Date:   Thu, 26 Mar 2020 11:09:16 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Michel Lespinasse <walken@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Liam Howlett <Liam.Howlett@...cle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        David Rientjes <rientjes@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 1/8] mmap locking API: initial implementation as rwsem
 wrappers

On Thu, Mar 26, 2020 at 11:06:21AM -0700, Matthew Wilcox wrote:
> On Thu, Mar 26, 2020 at 02:56:44PM -0300, Jason Gunthorpe wrote:
> > On Thu, Mar 26, 2020 at 12:02:29AM -0700, Michel Lespinasse wrote:
> > 
> > > +static inline bool mmap_is_locked(struct mm_struct *mm)
> > > +{
> > > +	return rwsem_is_locked(&mm->mmap_sem) != 0;
> > > +}
> > 
> > I've been wondering if the various VM_BUG(rwsem_is_locked()) would be
> > better as lockdep expressions? Certainly when lockdep is enabled it
> > should be preferred, IMHO.
> > 
> > So, I think if inlines are to be introduced this should be something
> > similar to netdev's ASSERT_RTNL which seems to have worked well.
> > 
> > Maybe ASSERT_MMAP_SEM_READ/WRITE/HELD() and do the VM_BUG or
> > lockdep_is_held as appropriate?
> 
> I'd rather see lockdep_assert_held() used directly rather than have
> a wrapper.  This API includes options to check for it beind explicitly
> held for read/write/any, which should be useful.

... oh, but that requires naming the lock, which we're trying to get
away from.

I guess we need a wrapper, but yes, by all means, let's level it up
to put the VM_BUG_ON inside the wrapper, as that means we can get the
mm dumped everywhere, rather than just the few places that have done
VM_BUG_ON_MM instead of plain VM_BUG_ON.