linux-kernel - Re: [RFC][PATCH 5/9] mm: Rename pmd_read

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Message-ID: <20201201085722.GR2414@hirez.programming.kicks-ass.net>
Date:   Tue, 1 Dec 2020 09:57:22 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     x86@...nel.org, willy@...radead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, aarcange@...hat.com,
        kirill.shutemov@...ux.intel.com, jroedel@...e.de
Subject: Re: [RFC][PATCH 5/9] mm: Rename pmd_read_atomic()

On Mon, Nov 30, 2020 at 11:31:20AM -0400, Jason Gunthorpe wrote:
> On Mon, Nov 30, 2020 at 12:27:10PM +0100, Peter Zijlstra wrote:
> > There's no point in having the identical routines for PTE/PMD have
> > different names.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> >  include/linux/pgtable.h    |    7 +------
> >  mm/hmm.c                   |    2 +-
> >  mm/mapping_dirty_helpers.c |    2 +-
> >  mm/mprotect.c              |    2 +-
> >  mm/userfaultfd.c           |    2 +-
> >  5 files changed, 5 insertions(+), 10 deletions(-)
> > 
> > +++ b/include/linux/pgtable.h
> > @@ -1244,11 +1244,6 @@ static inline int pud_trans_unstable(pud
> >  #endif
> >  }
> >  
> > -static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
> > -{
> > -	return pmdp_get_lockless(pmdp);
> > -}
> > -
> >  #ifndef arch_needs_pgtable_deposit
> >  #define arch_needs_pgtable_deposit() (false)
> >  #endif
> > @@ -1275,7 +1270,7 @@ static inline pmd_t pmd_read_atomic(pmd_
> >   */
> >  static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)
> >  {
> > -	pmd_t pmdval = pmd_read_atomic(pmd);
> > +	pmd_t pmdval = pmdp_get_lockless(pmd);
> >  	/*
> >  	 * The barrier will stabilize the pmdval in a register or on
> >  	 * the stack so that it will stop changing under the code.
> > +++ b/mm/hmm.c
> > @@ -356,7 +356,7 @@ static int hmm_vma_walk_pmd(pmd_t *pmdp,
> >  		 * huge or device mapping one and compute corresponding pfn
> >  		 * values.
> >  		 */
> > -		pmd = pmd_read_atomic(pmdp);
> > +		pmd = pmdp_get_lockless(pmdp);
> >  		barrier();
> >  		if (!pmd_devmap(pmd) && !pmd_trans_huge(pmd))
> >  			goto again;
> 
> The pagewalk API doesn't call the functions with interrupts disabled,
> doesn't this mean we hit this assertion?
> 
> +#if CONFIG_PGTABLE_LEVELS > 2
> +static inline pmd_t pmdp_get_lockless(pmd_t *pmdp)
> +{
> +       pmd_t pmd;
> +
> +       lockdep_assert_irqs_disabled();
> +
> 
> It is only holding the read side of the mmap_sem here

Hurmph, good point. I'll see what I can do about that.