Re: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem migration entry (fwd)

[Yang Shi]

On Tue, Jun 8, 2021 at 9:05 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
>
>
> ---------- Forwarded message ----------
> Date: Tue, 8 Jun 2021 21:00:12 -0700 (PDT)
> From: Hugh Dickins <hughd@xxxxxxxxxx>
> To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>,
>     Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>,
>     Yang Shi <shy828301@xxxxxxxxx>, Wang Yugui <wangyugui@xxxxxxxxxxxx>,
>     Matthew Wilcox <willy@xxxxxxxxxxxxx>,
>     Naoya Horiguchi <naoya.horiguchi@xxxxxxx>,
>     Alistair Popple <apopple@xxxxxxxxxx>, Ralph Campbell <rcampbell@xxxxxxxxxx>,
>     Zi Yan <ziy@xxxxxxxxxx>, Miaohe Lin <linmiaohe@xxxxxxxxxx>,
>     Minchan Kim <minchan@xxxxxxxxxx>, Jue Wang <juew@xxxxxxxxxx>,
>     Peter Xu <peterx@xxxxxxxxxx>, Jan Kara <jack@xxxxxxx>,
>     Shakeel Butt <shakeelb@xxxxxxxxxx>, Oscar Salvador <osalvador@xxxxxxx>
> Subject: [PATCH v2 01/10] mm/thp: fix __split_huge_pmd_locked() on shmem
>     migration entry
>
> Stressing huge tmpfs page migration racing hole punch often crashed on the
> VM_BUG_ON(!pmd_present) in pmdp_huge_clear_flush(), with DEBUG_VM=y kernel;
> or shortly afterwards, on a bad dereference in __split_huge_pmd_locked()
> when DEBUG_VM=n.  They forgot to allow for pmd migration entries in the
> non-anonymous case.
>
> Full disclosure: those particular experiments were on a kernel with more
> relaxed mmap_lock and i_mmap_rwsem locking, and were not repeated on the
> vanilla kernel: it is conceivable that stricter locking happens to avoid
> those cases, or makes them less likely; but __split_huge_pmd_locked()
> already allowed for pmd migration entries when handling anonymous THPs,
> so this commit brings the shmem and file THP handling into line.
>
> And while there: use old_pmd rather than _pmd, as in the following blocks;
> and make it clearer to the eye that the !vma_is_anonymous() block is
> self-contained, making an early return after accounting for unmapping.
>
> Fixes: e71769ae5260 ("mm: enable thp migration for shmem thp")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>

Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

> ---
> v2: omit is_huge_zero_pmd() mods (done differently in next), per Kirill
>
>  mm/huge_memory.c     | 27 ++++++++++++++++++---------
>  mm/pgtable-generic.c |  5 ++---
>  2 files changed, 20 insertions(+), 12 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63ed6b25deaa..42cfefc6e66e 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2044,7 +2044,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>         count_vm_event(THP_SPLIT_PMD);
>
>         if (!vma_is_anonymous(vma)) {
> -               _pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
> +               old_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
>                 /*
>                  * We are going to unmap this huge page. So
>                  * just go ahead and zap it
> @@ -2053,16 +2053,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                         zap_deposited_table(mm, pmd);
>                 if (vma_is_special_huge(vma))
>                         return;
> -               page = pmd_page(_pmd);
> -               if (!PageDirty(page) && pmd_dirty(_pmd))
> -                       set_page_dirty(page);
> -               if (!PageReferenced(page) && pmd_young(_pmd))
> -                       SetPageReferenced(page);
> -               page_remove_rmap(page, true);
> -               put_page(page);
> +               if (unlikely(is_pmd_migration_entry(old_pmd))) {
> +                       swp_entry_t entry;
> +
> +                       entry = pmd_to_swp_entry(old_pmd);
> +                       page = migration_entry_to_page(entry);
> +               } else {
> +                       page = pmd_page(old_pmd);
> +                       if (!PageDirty(page) && pmd_dirty(old_pmd))
> +                               set_page_dirty(page);
> +                       if (!PageReferenced(page) && pmd_young(old_pmd))
> +                               SetPageReferenced(page);
> +                       page_remove_rmap(page, true);
> +                       put_page(page);
> +               }
>                 add_mm_counter(mm, mm_counter_file(page), -HPAGE_PMD_NR);
>                 return;
> -       } else if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
> +       }
> +
> +       if (pmd_trans_huge(*pmd) && is_huge_zero_pmd(*pmd)) {
>                 /*
>                  * FIXME: Do we want to invalidate secondary mmu by calling
>                  * mmu_notifier_invalidate_range() see comments below inside
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index c2210e1cdb51..4e640baf9794 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -135,9 +135,8 @@ pmd_t pmdp_huge_clear_flush(struct vm_area_struct *vma, unsigned long address,
>  {
>         pmd_t pmd;
>         VM_BUG_ON(address & ~HPAGE_PMD_MASK);
> -       VM_BUG_ON(!pmd_present(*pmdp));
> -       /* Below assumes pmd_present() is true */
> -       VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp));
> +       VM_BUG_ON(pmd_present(*pmdp) && !pmd_trans_huge(*pmdp) &&
> +                          !pmd_devmap(*pmdp));
>         pmd = pmdp_huge_get_and_clear(vma->vm_mm, address, pmdp);
>         flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
>         return pmd;
> --
> 2.26.2
>
>