On Sun, Feb 05, 2017 at 11:12:41AM -0500, Zi Yan wrote:
> From: Zi Yan <ziy@xxxxxxxxxx>
>
> Originally, zap_pmd_range() checks pmd value without taking pmd lock.
> This can cause pmd_protnone entry not being freed.
>
> Because there are two steps in changing a pmd entry to a pmd_protnone
> entry. First, the pmd entry is cleared to a pmd_none entry, then,
> the pmd_none entry is changed into a pmd_protnone entry.
> The racy check, even with barrier, might only see the pmd_none entry
> in zap_pmd_range(), thus, the mapping is neither split nor zapped.
That's definately a good catch.
But I don't agree with the solution. Taking pmd lock on each
zap_pmd_range() is a significant hit by scalability of the code path.
Yes, split ptl lock helps, but it would be nice to avoid the lock in first
place.
Can we fix change_huge_pmd() instead? Is there a reason why we cannot
setup the pmd_protnone() atomically?
Mel? Rik?
>
> Later, in free_pmd_range(), pmd_none_or_clear() will see the
> pmd_protnone entry and clear it as a pmd_bad entry. Furthermore,
> since the pmd_protnone entry is not properly freed, the corresponding
> deposited pte page table is not freed either.
>
> This causes memory leak or kernel crashing, if VM_BUG_ON() is enabled.
>
> This patch relies on __split_huge_pmd_locked() and
> __zap_huge_pmd_locked().
>
> Signed-off-by: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> ---
> mm/memory.c | 24 +++++++++++-------------
> 1 file changed, 11 insertions(+), 13 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 3929b015faf7..7cfdd5208ef5 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1233,33 +1233,31 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
> struct zap_details *details)
> {
> pmd_t *pmd;
> + spinlock_t *ptl;
> unsigned long next;
>
> pmd = pmd_offset(pud, addr);
> + ptl = pmd_lock(vma->vm_mm, pmd);
> do {
> next = pmd_addr_end(addr, end);
> if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
> if (next - addr != HPAGE_PMD_SIZE) {
> VM_BUG_ON_VMA(vma_is_anonymous(vma) &&
> !rwsem_is_locked(&tlb->mm->mmap_sem), vma);
> - __split_huge_pmd(vma, pmd, addr, false, NULL);
> - } else if (zap_huge_pmd(tlb, vma, pmd, addr))
> - goto next;
> + __split_huge_pmd_locked(vma, pmd, addr, false);
> + } else if (__zap_huge_pmd_locked(tlb, vma, pmd, addr))
> + continue;
> /* fall through */
> }
> - /*
> - * Here there can be other concurrent MADV_DONTNEED or
> - * trans huge page faults running, and if the pmd is
> - * none or trans huge it can change under us. This is
> - * because MADV_DONTNEED holds the mmap_sem in read
> - * mode.
> - */
> - if (pmd_none_or_trans_huge_or_clear_bad(pmd))
> - goto next;
> +
> + if (pmd_none_or_clear_bad(pmd))
> + continue;
> + spin_unlock(ptl);
> next = zap_pte_range(tlb, vma, pmd, addr, next, details);
> -next:
> cond_resched();
> + spin_lock(ptl);
> } while (pmd++, addr = next, addr != end);
> + spin_unlock(ptl);
>
> return addr;
> }
> --
> 2.11.0
>
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--
Kirill A. Shutemov
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