On Fri, Apr 24, 2020 at 12:07:50PM +0800, Li Xinhai wrote: > 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) Sure > , so when sz == PMD_SIZE, pmd_offset() only called with a valid PUD > entry which point to PMD page table. But what prevents pud_huge? This API seems kind of strange to be honest.. Should it be two functions instead of a sz parameter? huge_pud_offset() and huge_pmd_offset() ? Jason