linux-kernel - Re: [PATCH 11/13] mm: cleanup the hmm_vma_handle

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Message-ID: <20190801070151.GB15404@lst.de>
Date:   Thu, 1 Aug 2019 09:01:51 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Christoph Hellwig <hch@....de>,
        Jérôme Glisse <jglisse@...hat.com>,
        Ben Skeggs <bskeggs@...hat.com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Ralph Campbell <rcampbell@...dia.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "amd-gfx@...ts.freedesktop.org" <amd-gfx@...ts.freedesktop.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/13] mm: cleanup the hmm_vma_handle_pmd stub

On Tue, Jul 30, 2019 at 05:53:14PM +0000, Jason Gunthorpe wrote:
> > -	/* If THP is not enabled then we should never reach this 
> 
> This old comment says we should never get here
> 
> > +}
> > +#else /* CONFIG_TRANSPARENT_HUGEPAGE */
> > +static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
> > +		unsigned long end, uint64_t *pfns, pmd_t pmd)
> > +{
> >  	return -EINVAL;
> 
> So could we just do
>    #define hmm_vma_handle_pmd NULL
> 
> ?
> 
> At the very least this seems like a WARN_ON too?

Despite the name of the function hmm_vma_handle_pmd is not a callback
for the pagewalk, but actually called from hmm_vma_handle_pmd.

What we could try is just and empty non-inline prototype without an
actual implementation, which means if the compiler doesn't optimize
the calls away we'll get a link error.