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From:   Yang Shi <shy828301@gmail.com>
Date:   Mon, 22 May 2023 16:35:36 -0700
Message-ID: <CAHbLzkosPTuxnkLRwukyUVNryj-Ci6_dbkM8MFFzn4q9bRmy2w@mail.gmail.com>
Subject: Re: [PATCH 26/31] mm/huge_memory: split huge pmd under one pte_offset_map()
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Qi Zheng <zhengqi.arch@bytedance.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Alistair Popple <apopple@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Steven Price <steven.price@arm.com>,
        SeongJae Park <sj@kernel.org>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Zack Rusin <zackr@vmware.com>, Jason Gunthorpe <jgg@ziepe.ca>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Minchan Kim <minchan@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Song Liu <song@kernel.org>,
        Thomas Hellstrom <thomas.hellstrom@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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On Sun, May 21, 2023 at 10:23=E2=80=AFPM Hugh Dickins <hughd@google.com> wr=
ote:
>
> __split_huge_zero_page_pmd() use a single pte_offset_map() to sweep the
> extent: it's already under pmd_lock(), so this is no worse for latency;
> and since it's supposed to have full control of the just-withdrawn page
> table, here choose to VM_BUG_ON if it were to fail.  And please don't
> increment haddr by PAGE_SIZE, that should remain huge aligned: declare
> a separate addr (not a bugfix, but it was deceptive).
>
> __split_huge_pmd_locked() likewise (but it had declared a separate addr);
> and change its BUG_ON(!pte_none) to VM_BUG_ON, for consistency with zero
> (those deposited page tables are sometimes victims of random corruption).
>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/huge_memory.c | 28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index d4bd5fa7c823..839c13fa0bbe 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2037,6 +2037,8 @@ static void __split_huge_zero_page_pmd(struct vm_ar=
ea_struct *vma,
>         struct mm_struct *mm =3D vma->vm_mm;
>         pgtable_t pgtable;
>         pmd_t _pmd, old_pmd;
> +       unsigned long addr;
> +       pte_t *pte;
>         int i;
>
>         /*
> @@ -2052,17 +2054,20 @@ static void __split_huge_zero_page_pmd(struct vm_=
area_struct *vma,
>         pgtable =3D pgtable_trans_huge_withdraw(mm, pmd);
>         pmd_populate(mm, &_pmd, pgtable);
>
> -       for (i =3D 0; i < HPAGE_PMD_NR; i++, haddr +=3D PAGE_SIZE) {
> -               pte_t *pte, entry;
> -               entry =3D pfn_pte(my_zero_pfn(haddr), vma->vm_page_prot);
> +       pte =3D pte_offset_map(&_pmd, haddr);
> +       VM_BUG_ON(!pte);
> +       for (i =3D 0, addr =3D haddr; i < HPAGE_PMD_NR; i++, addr +=3D PA=
GE_SIZE) {
> +               pte_t entry;
> +
> +               entry =3D pfn_pte(my_zero_pfn(addr), vma->vm_page_prot);
>                 entry =3D pte_mkspecial(entry);
>                 if (pmd_uffd_wp(old_pmd))
>                         entry =3D pte_mkuffd_wp(entry);
> -               pte =3D pte_offset_map(&_pmd, haddr);
>                 VM_BUG_ON(!pte_none(*pte));
> -               set_pte_at(mm, haddr, pte, entry);
> -               pte_unmap(pte);
> +               set_pte_at(mm, addr, pte, entry);
> +               pte++;
>         }
> +       pte_unmap(pte - 1);
>         smp_wmb(); /* make pte visible before pmd */
>         pmd_populate(mm, pmd, pgtable);
>  }
> @@ -2077,6 +2082,7 @@ static void __split_huge_pmd_locked(struct vm_area_=
struct *vma, pmd_t *pmd,
>         bool young, write, soft_dirty, pmd_migration =3D false, uffd_wp =
=3D false;
>         bool anon_exclusive =3D false, dirty =3D false;
>         unsigned long addr;
> +       pte_t *pte;
>         int i;
>
>         VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
> @@ -2205,8 +2211,10 @@ static void __split_huge_pmd_locked(struct vm_area=
_struct *vma, pmd_t *pmd,
>         pgtable =3D pgtable_trans_huge_withdraw(mm, pmd);
>         pmd_populate(mm, &_pmd, pgtable);
>
> +       pte =3D pte_offset_map(&_pmd, haddr);
> +       VM_BUG_ON(!pte);
>         for (i =3D 0, addr =3D haddr; i < HPAGE_PMD_NR; i++, addr +=3D PA=
GE_SIZE) {
> -               pte_t entry, *pte;
> +               pte_t entry;
>                 /*
>                  * Note that NUMA hinting access restrictions are not
>                  * transferred to avoid any possibility of altering
> @@ -2249,11 +2257,11 @@ static void __split_huge_pmd_locked(struct vm_are=
a_struct *vma, pmd_t *pmd,
>                                 entry =3D pte_mkuffd_wp(entry);
>                         page_add_anon_rmap(page + i, vma, addr, false);
>                 }
> -               pte =3D pte_offset_map(&_pmd, addr);
> -               BUG_ON(!pte_none(*pte));
> +               VM_BUG_ON(!pte_none(*pte));
>                 set_pte_at(mm, addr, pte, entry);
> -               pte_unmap(pte);
> +               pte++;
>         }
> +       pte_unmap(pte - 1);
>
>         if (!pmd_migration)
>                 page_remove_rmap(page, vma, true);
> --
> 2.35.3
>