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

["Li Xinhai" <lixinhai.lxh@gmail.com>]

On 2020-04-24 at 02:38 Jason Gunthorpe wrote:
>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. 

The test of !pud_present(*pud) also block pud_none(*pud), so when sz ==
PMD_SIZE, pmd_offset() only called with a valid PUD entry which point to PMD
page table.

>
>Jason