Re: [Nouveau] [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap

From: Jason Gunthorpe <jgg@nvidia.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: rcampbell@nvidia.com, willy@infradead.org,
	linux-doc@vger.kernel.org, nouveau@lists.freedesktop.org,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	kvm-ppc@vger.kernel.org, hch@infradead.org, linux-mm@kvack.org,
	bskeggs@redhat.com, daniel@ffwll.ch, akpm@linux-foundation.org,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [Nouveau] [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap
Date: Tue, 30 Mar 2021 19:24:40 -0300	[thread overview]
Message-ID: <20210330222440.GC2356281@nvidia.com> (raw)
In-Reply-To: <12442194.rtmf8Ope3M@nvdebian>

On Wed, Mar 31, 2021 at 09:09:30AM +1100, Alistair Popple wrote:

> > > @@ -1796,8 +1821,7 @@ bool try_to_unmap(struct page *page, enum ttu_flags 
> flags)
> > >  void try_to_munlock(struct page *page)
> > >  {
> > 
> > But this is also called try_to_munlock ??
> 
> As far as I can tell this has always been called try_to_munlock() even though 
> it appears to do the opposite.

Maybe we should change it then?

> > /**
> >  * try_to_munlock - try to munlock a page
> >  * @page: the page to be munlocked
> >  *
> >  * Called from munlock code.  Checks all of the VMAs mapping the page
> >  * to make sure nobody else has this page mlocked. The page will be
> >  * returned with PG_mlocked cleared if no other vmas have it mlocked.
> >  */
> 
> In other words it sets PG_mlocked if one or more vmas has it mlocked. So
> try_to_mlock() might be a better name, except that seems to have the potential 
> for confusion as well because it's only called from the munlock code path and 
> never for mlock.

That explanation makes more sense.. This function looks like it is
'set PG_mlocked of the page if any vm->flags has VM_LOCKED'

Maybe call it check_vm_locked or something then and reword the above
comment?

(and why is it OK to read vm->flags for this without any locking?)

> > Something needs attention here..
> 
> I think the code is correct, but perhaps the naming could be better. Would be 
> interested hearing any thoughts on renaming try_to_munlock() to try_to_mlock() 
> as the current name appears based on the context it is called from (munlock) 
> rather than what it does (mlock).

The point of this patch is to make it clearer, after all, so I'd
change something and maybe slightly clarify the comment.

Jason
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