linux-kernel - Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to

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Message-ID: <20210330222440.GC2356281@nvidia.com>
Date:   Tue, 30 Mar 2021 19:24:40 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, nouveau@...ts.freedesktop.org,
        bskeggs@...hat.com, akpm@...ux-foundation.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        jhubbard@...dia.com, rcampbell@...dia.com, jglisse@...hat.com,
        hch@...radead.org, daniel@...ll.ch, willy@...radead.org,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v7 3/8] mm/rmap: Split try_to_munlock from try_to_unmap

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