Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV_DONTNEED processing

[Mike Kravetz]

On 10/26/22 17:42, Peter Xu wrote:
> Hi, Mike,
> 
> On Sat, Oct 22, 2022 at 07:50:47PM -0700, Mike Kravetz wrote:
> 
> [...]
> 
> > -void __unmap_hugepage_range_final(struct mmu_gather *tlb,
> > +static void __unmap_hugepage_range_locking(struct mmu_gather *tlb,
> >  			  struct vm_area_struct *vma, unsigned long start,
> >  			  unsigned long end, struct page *ref_page,
> > -			  zap_flags_t zap_flags)
> > +			  zap_flags_t zap_flags, bool final)
> >  {
> >  	hugetlb_vma_lock_write(vma);
> >  	i_mmap_lock_write(vma->vm_file->f_mapping);
> >  
> >  	__unmap_hugepage_range(tlb, vma, start, end, ref_page, zap_flags);
> >  
> > -	/*
> > -	 * Unlock and free the vma lock before releasing i_mmap_rwsem.  When
> > -	 * the vma_lock is freed, this makes the vma ineligible for pmd
> > -	 * sharing.  And, i_mmap_rwsem is required to set up pmd sharing.
> > -	 * This is important as page tables for this unmapped range will
> > -	 * be asynchrously deleted.  If the page tables are shared, there
> > -	 * will be issues when accessed by someone else.
> > -	 */
> > -	__hugetlb_vma_unlock_write_free(vma);
> > +	if (final) {
> > +		/*
> > +		 * Unlock and free the vma lock before releasing i_mmap_rwsem.
> > +		 * When the vma_lock is freed, this makes the vma ineligible
> > +		 * for pmd sharing.  And, i_mmap_rwsem is required to set up
> > +		 * pmd sharing.  This is important as page tables for this
> > +		 * unmapped range will be asynchrously deleted.  If the page
> > +		 * tables are shared, there will be issues when accessed by
> > +		 * someone else.
> > +		 */
> > +		__hugetlb_vma_unlock_write_free(vma);
> > +		i_mmap_unlock_write(vma->vm_file->f_mapping);
> 
> Pure question: can we rely on hugetlb_vm_op_close() to destroy the hugetlb
> vma lock?
> 
> I read the comment above, it seems we are trying to avoid racing with pmd
> sharing, but I don't see how that could ever happen, since iiuc there
> should only be two places that unmaps the vma (final==true):
> 
>   (1) munmap: we're holding write lock, so no page fault possible
>   (2) exit_mmap: we've already reset current->mm so no page fault possible
> 

Thanks for taking a look Peter!

The possible sharing we are trying to stop would be initiated by a fault
in a different process on the same underlying mapping object (inode).  The
specific vma in exit processing is still linked into the mapping interval
tree.  So, even though we call huge_pmd_unshare in the unmap processing (in
__unmap_hugepage_range) the sharing could later be initiated by another
process.

Hope that makes sense.  That is also the reason the routine
page_table_shareable contains this check:

	/*
	 * match the virtual addresses, permission and the alignment of the
	 * page table page.
	 *
	 * Also, vma_lock (vm_private_data) is required for sharing.
	 */
	if (pmd_index(addr) != pmd_index(saddr) ||
	    vm_flags != svm_flags ||
	    !range_in_vma(svma, sbase, s_end) ||
	    !svma->vm_private_data)
		return 0;

FYI - The 'flags' check also prevents a non-uffd mapping from initiating
sharing with a uffd mapping.

> > +	} else {
> > +		i_mmap_unlock_write(vma->vm_file->f_mapping);
> > +		hugetlb_vma_unlock_write(vma);
> > +	}
> > +}
> >  
> > -	i_mmap_unlock_write(vma->vm_file->f_mapping);
> > +void __unmap_hugepage_range_final(struct mmu_gather *tlb,
> > +			  struct vm_area_struct *vma, unsigned long start,
> > +			  unsigned long end, struct page *ref_page,
> > +			  zap_flags_t zap_flags)
> > +{
> > +	__unmap_hugepage_range_locking(tlb, vma, start, end, ref_page,
> > +					zap_flags, true);
> >  }
> >  
> > +#ifdef CONFIG_ADVISE_SYSCALLS
> > +/*
> > + * Similar setup as in zap_page_range().  madvise(MADV_DONTNEED) can not call
> > + * zap_page_range for hugetlb vmas as __unmap_hugepage_range_final will delete
> > + * the associated vma_lock.
> > + */
> > +void clear_hugetlb_page_range(struct vm_area_struct *vma, unsigned long start,
> > +				unsigned long end)
> > +{
> > +	struct mmu_notifier_range range;
> > +	struct mmu_gather tlb;
> > +
> > +	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
> > +				start, end);
> 
> Is mmu_notifier_invalidate_range_start() missing here?
> 

It certainly does look like it.  When I created this routine, I was trying to
mimic what was done in the current calling path zap_page_range to
__unmap_hugepage_range_final.  Now when I look at that, I am not seeing
a mmu_notifier_invalidate_range_start/end.  Am I missing something, or
are these missing today?  Do note that we do MMU_NOTIFY_UNMAP in
__unmap_hugepage_range.

> > +	tlb_gather_mmu(&tlb, vma->vm_mm);
> > +	update_hiwater_rss(vma->vm_mm);
> > +
> > +	__unmap_hugepage_range_locking(&tlb, vma, start, end, NULL, 0, false);
> > +
> > +	mmu_notifier_invalidate_range_end(&range);
> > +	tlb_finish_mmu(&tlb);
> > +}
> > +#endif
> > +
> >  void unmap_hugepage_range(struct vm_area_struct *vma, unsigned long start,
> >  			  unsigned long end, struct page *ref_page,
> >  			  zap_flags_t zap_flags)
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 2baa93ca2310..90577a669635 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -790,7 +790,10 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
> >  static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
> >  					unsigned long start, unsigned long end)
> >  {
> > -	zap_page_range(vma, start, end - start);
> > +	if (!is_vm_hugetlb_page(vma))
> > +		zap_page_range(vma, start, end - start);
> > +	else
> > +		clear_hugetlb_page_range(vma, start, end);
> >  	return 0;
> >  }
> 
> This does look a bit unfortunate - zap_page_range() contains yet another
> is_vm_hugetlb_page() check (further down in unmap_single_vma), it can be
> very confusing on which code path is really handling hugetlb.
> 
> The other mm_users check in v3 doesn't need this change, but was a bit
> hackish to me, because IIUC we're clear on the call paths to trigger this
> (unmap_vmas), so it seems clean to me to pass that info from the upper
> stack.
> 
> Maybe we can have a new zap_flags passed into unmap_single_vma() showing
> that it's destroying the vma?

I thought about that.  However, we would need to start passing the flag
here into zap_page_range as this is the beginning of that call down into
the hugetlb code where we do not want to remove zap_page_rangethe
vma_lock.

-- 
Mike Kravetz