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Date:   Thu, 7 Jul 2022 16:03:44 -0700
From:   Mike Kravetz <mike.kravetz@oracle.com>
To:     "manish.mishra" <manish.mishra@nutanix.com>
Cc:     James Houghton <jthoughton@google.com>,
        Muchun Song <songmuchun@bytedance.com>,
        Peter Xu <peterx@redhat.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        Jue Wang <juew@google.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC PATCH 11/26] hugetlb: add hugetlb_walk_to to do PT walks
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On 06/27/22 18:37, manish.mishra wrote:
> 
> On 24/06/22 11:06 pm, James Houghton wrote:
> > This adds it for architectures that use GENERAL_HUGETLB, including x86.

I expect this will be used in arch independent code and there will need to
be at least a stub for all architectures?

> > 
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> >   include/linux/hugetlb.h |  2 ++
> >   mm/hugetlb.c            | 45 +++++++++++++++++++++++++++++++++++++++++
> >   2 files changed, 47 insertions(+)
> > 
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index e7a6b944d0cc..605aa19d8572 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -258,6 +258,8 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
> >   			unsigned long addr, unsigned long sz);
> >   pte_t *huge_pte_offset(struct mm_struct *mm,
> >   		       unsigned long addr, unsigned long sz);
> > +int hugetlb_walk_to(struct mm_struct *mm, struct hugetlb_pte *hpte,
> > +		    unsigned long addr, unsigned long sz, bool stop_at_none);
> >   int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma,
> >   				unsigned long *addr, pte_t *ptep);
> >   void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 557b0afdb503..3ec2a921ee6f 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6981,6 +6981,51 @@ pte_t *huge_pte_offset(struct mm_struct *mm,
> >   	return (pte_t *)pmd;
> >   }
> 
> 
> not strong feeling but this name looks confusing to me as it does
> 
> not only walk over page-tables but can also alloc.
> 

Somewhat agree.  With this we have:
- huge_pte_offset to walk/lookup a pte
- huge_pte_alloc to allocate ptes
- hugetlb_walk_to which does some/all of both

Do not see anything obviously wrong with the routine, but future
direction would be to combine/clean up these routines with similar
purpose.
-- 
Mike Kravetz

> > +int hugetlb_walk_to(struct mm_struct *mm, struct hugetlb_pte *hpte,
> > +		    unsigned long addr, unsigned long sz, bool stop_at_none)
> > +{
> > +	pte_t *ptep;
> > +
> > +	if (!hpte->ptep) {
> > +		pgd_t *pgd = pgd_offset(mm, addr);
> > +
> > +		if (!pgd)
> > +			return -ENOMEM;
> > +		ptep = (pte_t *)p4d_alloc(mm, pgd, addr);
> > +		if (!ptep)
> > +			return -ENOMEM;
> > +		hugetlb_pte_populate(hpte, ptep, P4D_SHIFT);
> > +	}
> > +
> > +	while (hugetlb_pte_size(hpte) > sz &&
> > +			!hugetlb_pte_present_leaf(hpte) &&
> > +			!(stop_at_none && hugetlb_pte_none(hpte))) {
> 
> Should this ordering of if-else condition be in reverse, i mean it will look
> 
> more natural and possibly less condition checks as we go from top to bottom.
> 
> > +		if (hpte->shift == PMD_SHIFT) {
> > +			ptep = pte_alloc_map(mm, (pmd_t *)hpte->ptep, addr);
> > +			if (!ptep)
> > +				return -ENOMEM;
> > +			hpte->shift = PAGE_SHIFT;
> > +			hpte->ptep = ptep;
> > +		} else if (hpte->shift == PUD_SHIFT) {
> > +			ptep = (pte_t *)pmd_alloc(mm, (pud_t *)hpte->ptep,
> > +						  addr);
> > +			if (!ptep)
> > +				return -ENOMEM;
> > +			hpte->shift = PMD_SHIFT;
> > +			hpte->ptep = ptep;
> > +		} else if (hpte->shift == P4D_SHIFT) {
> > +			ptep = (pte_t *)pud_alloc(mm, (p4d_t *)hpte->ptep,
> > +						  addr);
> > +			if (!ptep)
> > +				return -ENOMEM;
> > +			hpte->shift = PUD_SHIFT;
> > +			hpte->ptep = ptep;
> > +		} else
> > +			BUG();
> > +	}
> > +	return 0;
> > +}
> > +
> >   #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */
> >   #ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING