Subject: Re: [PATCH v2 1/2] secretmem/gup: don't check if page is secretmem without reference

From: Mike Rapoport <[email-masked]>
Date: Tue, 20 Apr 2021 16:27:56 +0300

On Tue, Apr 20, 2021 at 03:19:56PM +0200, David Hildenbrand wrote:
> On 20.04.21 15:16, Mike Rapoport wrote:
> > From: Mike Rapoport <[email-masked]>
> > 
> > The check in gup_pte_range() whether a page belongs to a secretmem mapping
> > is performed before grabbing the page reference.
> > 
> > To avoid potential race move the check after try_grab_compound_head().
> > 
> > Signed-off-by: Mike Rapoport <[email-masked]>
> > ---
> >   mm/gup.c | 6 +++---
> >   1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/gup.c b/mm/gup.c
> > index c3a17b189064..4b58c016e949 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2080,13 +2080,13 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >   		VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
> >   		page = pte_page(pte);
> > -		if (page_is_secretmem(page))
> > -			goto pte_unmap;
> > -
> >   		head = try_grab_compound_head(page, 1, flags);
> >   		if (!head)
> >   			goto pte_unmap;
> > +		if (page_is_secretmem(page))
> > +			goto pte_unmap;
> > +
> 
> Looking at the hunk below, I wonder if you're missing a put_compound_head().

Hmm, yes.
 
> (also, I'd do if unlikely(page_is_secretmem()) but that's a different
> discussion)

I don't mind, actually. I don't think there would be massive secretmem
usage soon.

> >   		if (unlikely(pte_val(pte) != pte_val(*ptep))) {
> >   			put_compound_head(head, 1, flags);
> >   			goto pte_unmap;
> > 
> 
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.

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