linux-kernel - Re: [PATCH] mm/gup: dereference page table entry using helper

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Message-ID: <20200414185829.GB1853609@iweiny-DESK2.sc.intel.com>
Date:   Tue, 14 Apr 2020 11:58:29 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm/gup: dereference page table entry using helper

On Tue, Apr 14, 2020 at 01:32:34PM -0300, Jason Gunthorpe wrote:
> On Tue, Apr 14, 2020 at 05:10:01PM +0200, Alexander Gordeev wrote:
> > Commit 0005d20 ("mm/gup: Move page table entry dereference
> > into helper function") wrapped access to page table entries
> > larger than sizeof(long) into a race-aware accessor. One of
> > the two dereferences in gup_fast path was however overlooked.
> > 
> > CC: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > CC: linux-mm@...ck.org
> > Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
> >  mm/gup.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index d53f7dd..eceb98b 100644
> > +++ b/mm/gup.c
> > @@ -2208,7 +2208,7 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >  		if (!head)
> >  			goto pte_unmap;
> >  
> > -		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> > +		if (unlikely(pte_val(pte) != pte_val(gup_get_pte(ptep)))) {
> 
> It doesn't seem like this needs the special helper as it is just
> checking that the pte hasn't changed, it doesn't need to be read
> exactly.
> 
> But it probably should technically still be a READ_ONCE. Although I
> think the atomic inside try_grab_compound_head prevents any real
> problems.

I think we should go for consistency here and use the helper function.

Ira

> 
> Jason
>