On Fri, Nov 02, 2018 at 01:22:42PM +0000, Will Deacon wrote:
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>
> Commit eb66ae030829605d61fbef1909ce310e29f78821 upstream.
I have never seen the original patch on mailing lists, so I'll reply to
the backport.
>
> This is a backport to stable 4.4.y.
>
> Jann Horn points out that our TLB flushing was subtly wrong for the
> mremap() case. What makes mremap() special is that we don't follow the
> usual "add page to list of pages to be freed, then flush tlb, and then
> free pages". No, mremap() obviously just _moves_ the page from one page
> table location to another.
>
> That matters, because mremap() thus doesn't directly control the
> lifetime of the moved page with a freelist: instead, the lifetime of the
> page is controlled by the page table locking, that serializes access to
> the entry.
I believe we do control the lifetime of the page with mmap_sem, don't we?
I mean any shoot down of the page from a mapping would require at least
down_read(mmap_sem) and we hold down_write(mmap_sem). Hm?
> As a result, we need to flush the TLB not just before releasing the lock
> for the source location (to avoid any concurrent accesses to the entry),
> but also before we release the destination page table lock (to avoid the
> TLB being flushed after somebody else has already done something to that
> page).
>
> This also makes the whole "need_flush" logic unnecessary, since we now
> always end up flushing the TLB for every valid entry.
>
> Reported-and-tested-by: Jann Horn <jannh@xxxxxxxxxx>
> Acked-by: Will Deacon <will.deacon@xxxxxxx>
> Tested-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> [will: backport to 4.4 stable]
> Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> ---
> mm/huge_memory.c | 6 +++++-
> mm/mremap.c | 21 ++++++++++++++++-----
> 2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index c4ea57ee2fd1..465786cd6490 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1511,7 +1511,7 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
> spinlock_t *old_ptl, *new_ptl;
> int ret = 0;
> pmd_t pmd;
> -
> + bool force_flush = false;
> struct mm_struct *mm = vma->vm_mm;
>
> if ((old_addr & ~HPAGE_PMD_MASK) ||
> @@ -1539,6 +1539,8 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
> if (new_ptl != old_ptl)
> spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
> + if (pmd_present(pmd))
> + force_flush = true;
> VM_BUG_ON(!pmd_none(*new_pmd));
>
> if (pmd_move_must_withdraw(new_ptl, old_ptl)) {
> @@ -1547,6 +1549,8 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma,
> pgtable_trans_huge_deposit(mm, new_pmd, pgtable);
> }
> set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd));
> + if (force_flush)
> + flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
> if (new_ptl != old_ptl)
> spin_unlock(new_ptl);
> spin_unlock(old_ptl);
> diff --git a/mm/mremap.c b/mm/mremap.c
> index fe7b7f65f4f4..450b306d473e 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -96,6 +96,8 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
> struct mm_struct *mm = vma->vm_mm;
> pte_t *old_pte, *new_pte, pte;
> spinlock_t *old_ptl, *new_ptl;
> + bool force_flush = false;
> + unsigned long len = old_end - old_addr;
>
> /*
> * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
> @@ -143,12 +145,26 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
> if (pte_none(*old_pte))
> continue;
> pte = ptep_get_and_clear(mm, old_addr, old_pte);
> + /*
> + * If we are remapping a valid PTE, make sure
> + * to flush TLB before we drop the PTL for the PTE.
> + *
> + * NOTE! Both old and new PTL matter: the old one
> + * for racing with page_mkclean(), the new one to
> + * make sure the physical page stays valid until
> + * the TLB entry for the old mapping has been
> + * flushed.
> + */
Could you elaborate on the race with page_mkclean()?
I think the new logic is unnecessary strict (and slow).
Any barely sane userspace must not access the old mapping after
mremap(MREMAP_MAYMOVE) called and must not access the new mapping
before the mremap() returns.
The old logic *should* be safe if this argument valid.
Do I miss something?
> + if (pte_present(pte))
> + force_flush = true;
> pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
> pte = move_soft_dirty_pte(pte);
> set_pte_at(mm, new_addr, new_pte, pte);
> }
>
> arch_leave_lazy_mmu_mode();
> + if (force_flush)
> + flush_tlb_range(vma, old_end - len, old_end);
> if (new_ptl != old_ptl)
> spin_unlock(new_ptl);
> pte_unmap(new_pte - 1);
> @@ -168,7 +184,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> {
> unsigned long extent, next, old_end;
> pmd_t *old_pmd, *new_pmd;
> - bool need_flush = false;
> unsigned long mmun_start; /* For mmu_notifiers */
> unsigned long mmun_end; /* For mmu_notifiers */
>
> @@ -207,7 +222,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> anon_vma_unlock_write(vma->anon_vma);
> }
> if (err > 0) {
> - need_flush = true;
> continue;
> } else if (!err) {
> split_huge_page_pmd(vma, old_addr, old_pmd);
> @@ -224,10 +238,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> extent = LATENCY_LIMIT;
> move_ptes(vma, old_pmd, old_addr, old_addr + extent,
> new_vma, new_pmd, new_addr, need_rmap_locks);
> - need_flush = true;
> }
> - if (likely(need_flush))
> - flush_tlb_range(vma, old_end-len, old_addr);
>
> mmu_notifier_invalidate_range_end(vma->vm_mm, mmun_start, mmun_end);
>
> --
> 2.1.4
>
--
Kirill A. Shutemov
