Re: [PATCH v3 4/9] hugetlb/userfaultfd: Unshare all pmds for hugetlbfs when register wp

[Mike Kravetz]

On 1/28/21 2:48 PM, Axel Rasmussen wrote:
> From: Peter Xu <peterx@xxxxxxxxxx>
> 
> Huge pmd sharing for hugetlbfs is racy with userfaultfd-wp because
> userfaultfd-wp is always based on pgtable entries, so they cannot be shared.
> 
> Walk the hugetlb range and unshare all such mappings if there is, right before
> UFFDIO_REGISTER will succeed and return to userspace.
> 
> This will pair with want_pmd_share() in hugetlb code so that huge pmd sharing
> is completely disabled for userfaultfd-wp registered range.
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> ---
>  fs/userfaultfd.c             | 45 ++++++++++++++++++++++++++++++++++++
>  include/linux/mmu_notifier.h |  1 +
>  2 files changed, 46 insertions(+)
> 
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 894cc28142e7..2c6706ac2504 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -15,6 +15,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/sched/mm.h>
>  #include <linux/mm.h>
> +#include <linux/mmu_notifier.h>
>  #include <linux/poll.h>
>  #include <linux/slab.h>
>  #include <linux/seq_file.h>
> @@ -1190,6 +1191,47 @@ static ssize_t userfaultfd_read(struct file *file, char __user *buf,
>  	}
>  }
>  
> +/*
> + * This function will unconditionally remove all the shared pmd pgtable entries
> + * within the specific vma for a hugetlbfs memory range.
> + */
> +static void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
> +{
> +#ifdef CONFIG_HUGETLB_PAGE
> +	struct hstate *h = hstate_vma(vma);
> +	unsigned long sz = huge_page_size(h);
> +	struct mm_struct *mm = vma->vm_mm;
> +	struct mmu_notifier_range range;
> +	unsigned long address;
> +	spinlock_t *ptl;
> +	pte_t *ptep;
> +

Perhaps we should add a quick to see if vma is sharable.  Might be as
simple as !(vma->vm_flags & VM_MAYSHARE).  I see a comment/question in
a later patch about only doing minor fault processing on shared mappings.

Code below looks fine, but it would be a wast to do all that for a vma
that could not be shared.

-- 
Mike Kravetz

> +	/*
> +	 * No need to call adjust_range_if_pmd_sharing_possible(), because
> +	 * we're going to operate on the whole vma
> +	 */
> +	mmu_notifier_range_init(&range, MMU_NOTIFY_HUGETLB_UNSHARE,
> +				0, vma, mm, vma->vm_start, vma->vm_end);
> +	mmu_notifier_invalidate_range_start(&range);
> +	i_mmap_lock_write(vma->vm_file->f_mapping);
> +	for (address = vma->vm_start; address < vma->vm_end; address += sz) {
> +		ptep = huge_pte_offset(mm, address, sz);
> +		if (!ptep)
> +			continue;
> +		ptl = huge_pte_lock(h, mm, ptep);
> +		huge_pmd_unshare(mm, vma, &address, ptep);
> +		spin_unlock(ptl);
> +	}
> +	flush_hugetlb_tlb_range(vma, vma->vm_start, vma->vm_end);
> +	i_mmap_unlock_write(vma->vm_file->f_mapping);
> +	/*
> +	 * No need to call mmu_notifier_invalidate_range(), see
> +	 * Documentation/vm/mmu_notifier.rst.
> +	 */
> +	mmu_notifier_invalidate_range_end(&range);
> +#endif
> +}
> +
>  static void __wake_userfault(struct userfaultfd_ctx *ctx,
>  			     struct userfaultfd_wake_range *range)
>  {
> @@ -1448,6 +1490,9 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
>  		vma->vm_flags = new_flags;
>  		vma->vm_userfaultfd_ctx.ctx = ctx;
>  
> +		if (is_vm_hugetlb_page(vma) && uffd_disable_huge_pmd_share(vma))
> +			hugetlb_unshare_all_pmds(vma);
> +
>  	skip:
>  		prev = vma;
>  		start = vma->vm_end;
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index b8200782dede..ff50c8528113 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -51,6 +51,7 @@ enum mmu_notifier_event {
>  	MMU_NOTIFY_SOFT_DIRTY,
>  	MMU_NOTIFY_RELEASE,
>  	MMU_NOTIFY_MIGRATE,
> +	MMU_NOTIFY_HUGETLB_UNSHARE,
>  };
>  
>  #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
>