linux-kernel - Re: [PATCH 03/11] mm: page_vma_mapped_walk(): use pmd_read

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Message-ID: <20210610121542.GQ1096940@ziepe.ca>
Date:   Thu, 10 Jun 2021 09:15:42 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>,
        Wang Yugui <wangyugui@...-tech.com>,
        Matthew Wilcox <willy@...radead.org>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>, Zi Yan <ziy@...dia.com>,
        Peter Xu <peterx@...hat.com>, Will Deacon <will@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] mm: page_vma_mapped_walk(): use pmd_read_atomic()

On Thu, Jun 10, 2021 at 12:06:17PM +0300, Kirill A. Shutemov wrote:
> On Wed, Jun 09, 2021 at 11:38:11PM -0700, Hugh Dickins wrote:
> > page_vma_mapped_walk() cleanup: use pmd_read_atomic() with barrier()
> > instead of READ_ONCE() for pmde: some architectures (e.g. i386 with PAE)
> > have a multi-word pmd entry, for which READ_ONCE() is not good enough.
> > 
> > Signed-off-by: Hugh Dickins <hughd@...gle.com>
> > Cc: <stable@...r.kernel.org>
> >  mm/page_vma_mapped.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> > index 7c0504641fb8..973c3c4e72cc 100644
> > +++ b/mm/page_vma_mapped.c
> > @@ -182,13 +182,16 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw)
> >  	pud = pud_offset(p4d, pvmw->address);
> >  	if (!pud_present(*pud))
> >  		return false;
> > +
> >  	pvmw->pmd = pmd_offset(pud, pvmw->address);
> >  	/*
> >  	 * Make sure the pmd value isn't cached in a register by the
> >  	 * compiler and used as a stale value after we've observed a
> >  	 * subsequent update.
> >  	 */
> > -	pmde = READ_ONCE(*pvmw->pmd);
> > +	pmde = pmd_read_atomic(pvmw->pmd);
> > +	barrier();
> > +
> 
> Hm. It makes me wounder if barrier() has to be part of pmd_read_atomic().
> mm/hmm.c uses the same pattern as you are and I tend to think that the
> rest of pmd_read_atomic() users may be broken.
> 
> Am I wrong?

I agree with you, something called _atomic should not require the
caller to provide barriers.

I think the issue is simply that the two implementations of
pmd_read_atomic() should use READ_ONCE() internally, no?

Jason