Re: [PATCH] mm/hugetlb: avoid unnecessary check on pud and pmd entry in huge_pte_offset

[Jason Gunthorpe <jgg@mellanox.com>]

On Thu, Apr 23, 2020 at 11:14:28AM -0700, Mike Kravetz wrote:
> Cc a few people who have looked at huge_pte_offset() recently.
> 
> On 4/23/20 5:49 AM, Li Xinhai wrote:
> > When huge_pte_offset() is called, the parameter sz can only be PUD_SIZE
> > or PMD_SIZE.
> > If sz is PUD_SIZE and code can reach pud, then *pud must be none, or
> > normal hugetlb entry, or non-present (migration or hwpoisoned) hugetlb
> > entry, and we can directly return pud.
> > When sz is PMD_SIZE, pud must be none or present, and if code can reach
> > pmd, we can directly return pmd.
> > 
> > So, after this patch, the code is simplified by first check on the
> > parameter sz, and avoid unnecessary checks in current code.
> > 
> > Signed-off-by: Li Xinhai <lixinhai.lxh@gmail.com>
> > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> >  mm/hugetlb.c | 24 +++++++++---------------
> >  1 file changed, 9 insertions(+), 15 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index bcabbe0..e1424f5 100644
> > +++ b/mm/hugetlb.c
> > @@ -5365,8 +5365,8 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> >  {
> >  	pgd_t *pgd;
> >  	p4d_t *p4d;
> > -	pud_t *pud, pud_entry;
> > -	pmd_t *pmd, pmd_entry;
> > +	pud_t *pud;
> > +	pmd_t *pmd;
> >  
> >  	pgd = pgd_offset(mm, addr);
> >  	if (!pgd_present(*pgd))
> > @@ -5376,22 +5376,16 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> >  		return NULL;
> >  
> >  	pud = pud_offset(p4d, addr);
> > -	pud_entry = READ_ONCE(*pud);
> > -	if (sz != PUD_SIZE && pud_none(pud_entry))
> > -		return NULL;
> > -	/* hugepage or swap? */
> > -	if (pud_huge(pud_entry) || !pud_present(pud_entry))
> > +	if (sz == PUD_SIZE)
> > +		/* must be pud_huge or pud_none */
> >  		return (pte_t *)pud;
> > -
> > -	pmd = pmd_offset(pud, addr);
> > -	pmd_entry = READ_ONCE(*pmd);
> > -	if (sz != PMD_SIZE && pmd_none(pmd_entry))
> > +	if (!pud_present(*pud))
> >  		return NULL;
> > -	/* hugepage or swap? */
> > -	if (pmd_huge(pmd_entry) || !pmd_present(pmd_entry))
> > -		return (pte_t *)pmd;
> > +	/* must have a valid entry and size to go further */
> >  
> > -	return NULL;
> > +	pmd = pmd_offset(pud, addr);
> 
> Can we get here with sz = PMD_SIZE and pud_none(*pud)?  Would that be
> an issue for the pmd_offset() call?

Certainly pmd_offset() must only be called if the PUD entry is
pointing at a pmd level.

AFAIK this means it should not be called on pud_none(), pud_huge() or
!pud_present() cases.

Jason