Re: [PATCH v8 1/2] hugetlb: don't delete vma_lock in hugetlb MADV_DONTNEED processing

Nadav Amit
2022-11-10T20:59:50Z

On Nov 7, 2022, at 5:19 PM, Mike Kravetz <mike.kravetz@oracle.com> wrote:

> madvise(MADV_DONTNEED) ends up calling zap_page_range() to clear page
> tables associated with the address range. For hugetlb vmas,
> zap_page_range will call __unmap_hugepage_range_final. However,
> __unmap_hugepage_range_final assumes the passed vma is about to be removed
> and deletes the vma_lock to prevent pmd sharing as the vma is on the way
> out. In the case of madvise(MADV_DONTNEED) the vma remains, but the
> missing vma_lock prevents pmd sharing and could potentially lead to issues
> with truncation/fault races.

I understand the problem in general. Please consider my feedback as partial
though.

> @@ -5203,32 +5194,50 @@ void __unmap_hugepage_range_final(struct mmu_gather *tlb,
> unsigned long end, struct page *ref_page,
> zap_flags_t zap_flags)
> {
> + bool final = zap_flags & ZAP_FLAG_UNMAP;
> +

Not sure why caching final in local variable helps.

>
> void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
> unsigned long end, struct page *ref_page,
> zap_flags_t zap_flags)
> {
> + struct mmu_notifier_range range;
> struct mmu_gather tlb;
>
> + mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
> + start, end);
> + adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
> tlb_gather_mmu(&tlb, vma->vm_mm);
> +
> __unmap_hugepage_range(&tlb, vma, start, end, ref_page, zap_flags);

Is there a reason for not using range.start and range.end?

It is just that every inconsistency is worrying…

>
> @@ -1734,6 +1734,9 @@ static void zap_page_range_single(struct vm_area_struct *vma, unsigned long addr
> lru_add_drain();
> mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> address, address + size);
> + if (is_vm_hugetlb_page(vma))
> + adjust_range_if_pmd_sharing_possible(vma, &range.start,
> + &range.end);
> tlb_gather_mmu(&tlb, vma->vm_mm);
> update_hiwater_rss(vma->vm_mm);
> mmu_notifier_invalidate_range_start(&range);
> @@ -1742,6 +1745,12 @@ static void zap_page_range_single(struct vm_area_struct *vma, unsigned long addr
> tlb_finish_mmu(&tlb);
> }
>
> +void zap_vma_range(struct vm_area_struct *vma, unsigned long address,
> + unsigned long size)
> +{
> + __zap_page_range_single(vma, address, size, NULL);

Ugh. So zap_vma_range() would actually be emitted as a wrapper function that
only calls __zap_page_range_single() (or worse __zap_page_range_single()
which is large would be inlined), unless you use LTO.

Another option is to declare __zap_page_range_size() in the header and move
this one to the header as inline wrapper.