Re: [PATCH v2 4/4] hugetlb/userfaultfd: Unshare all pmds for hugetlbfs when register wp

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 17, 2021 at 05:46:30PM -0800, Mike Kravetz wrote:
> On 2/17/21 12:46 PM, Peter Xu wrote:
> > 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@redhat.com>
> > ---
> >  fs/userfaultfd.c        |  4 ++++
> >  include/linux/hugetlb.h |  1 +
> >  mm/hugetlb.c            | 51 +++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 56 insertions(+)
> > 
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index 894cc28142e7..e259318fcae1 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>
> > @@ -1448,6 +1449,9 @@ 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/hugetlb.h b/include/linux/hugetlb.h
> > index 3b4104021dd3..97ecfd4c20b2 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -188,6 +188,7 @@ unsigned long hugetlb_change_protection(struct vm_area_struct *vma,
> >  		unsigned long address, unsigned long end, pgprot_t newprot);
> >  
> >  bool is_hugetlb_entry_migration(pte_t pte);
> > +void hugetlb_unshare_all_pmds(struct vm_area_struct *vma);
> >  
> >  #else /* !CONFIG_HUGETLB_PAGE */
> >  
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index f53a0b852ed8..83c006ea3ff9 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5723,4 +5723,55 @@ void __init hugetlb_cma_check(void)
> >  	pr_warn("hugetlb_cma: the option isn't supported by current arch\n");
> >  }
> >  
> > +/*
> > + * This function will unconditionally remove all the shared pmd pgtable entries
> > + * within the specific vma for a hugetlbfs memory range.
> > + */
> 
> Thanks for updating this!
> 
> > +void hugetlb_unshare_all_pmds(struct vm_area_struct *vma)
> > +{
> > +	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, start, end;
> > +	spinlock_t *ptl;
> > +	pte_t *ptep;
> > +
> > +	if (!(vma->vm_flags & VM_MAYSHARE))
> > +		return;
> > +
> > +	start = ALIGN(vma->vm_start, PUD_SIZE);
> > +	end = ALIGN_DOWN(vma->vm_end, PUD_SIZE);
> > +
> > +	if (start >= end)
> > +		return;
> > +
> > +	/*
> > +	 * No need to call adjust_range_if_pmd_sharing_possible(), because
> > +	 * we're going to operate on the whole vma
> 
> not necessary, but perhaps change to:
> 	 * we're going to operate on ever PUD_SIZE aligned sized range
> 	 * within the vma.
> 
> > +	 * we're going to operate on the whole vma
> > +	 */
> > +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, mm,
> > +				vma->vm_start, vma->vm_end);
> 
> Should we use start, end here instead of vma->vm_start, vma->vm_end ?
> 
> > +	mmu_notifier_invalidate_range_start(&range);
> > +	i_mmap_lock_write(vma->vm_file->f_mapping);
> > +	for (address = start; address < end; address += PUD_SIZE) {
> > +		unsigned long tmp = address;
> > +
> > +		ptep = huge_pte_offset(mm, address, sz);
> > +		if (!ptep)
> > +			continue;
> > +		ptl = huge_pte_lock(h, mm, ptep);
> > +		/* We don't want 'address' to be changed */
> > +		huge_pmd_unshare(mm, vma, &tmp, ptep);
> > +		spin_unlock(ptl);
> > +	}
> > +	flush_hugetlb_tlb_range(vma, vma->vm_start, vma->vm_end);
> 
> start, end ?

Right we can even shrink the notifier, I'll respin shortly.  Thanks,

-- 
Peter Xu