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@xxxxxxxxx>
>> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>> > 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
