Date:   Wed, 26 Oct 2022 21:12:50 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Chen <harperchen1110@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb
 MADV_DONTNEED processing

On Wed, Oct 26, 2022 at 04:54:01PM -0700, Mike Kravetz wrote:
> 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;

Ah, makes sense.  Hmm, then I'm wondering whether hugetlb_vma_lock_free()
would ever be useful at all?  Because remove_vma() (or say, the close()
hook) seems to always be called after an precedent unmap_vmas().

> 
> 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?

I'm not sure whether we're looking at the same code base; here it's in
zap_page_range() itself.

	mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
				start, start + size);
	tlb_gather_mmu(&tlb, vma->vm_mm);
	update_hiwater_rss(vma->vm_mm);
	mmu_notifier_invalidate_range_start(&range);
	do {
		unmap_single_vma(&tlb, vma, start, range.end, NULL);
	} while ((vma = mas_find(&mas, end - 1)) != NULL);
	mmu_notifier_invalidate_range_end(&range);

> Do note that we do MMU_NOTIFY_UNMAP in __unmap_hugepage_range.

Hmm, I think we may want CLEAR for zap-only and UNMAP only for unmap.

 * @MMU_NOTIFY_UNMAP: either munmap() that unmap the range or a mremap() that
 * move the range
 * @MMU_NOTIFY_CLEAR: clear page table entry (many reasons for this like
 * madvise() or replacing a page by another one, ...).

The other thing is that unmap_vmas() also notifies (same to
zap_page_range), it looks a duplicated notification if any of them calls
__unmap_hugepage_range() at last.

> 
> > > +	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.

Right.  I was thinking just attach the new flag in unmap_vmas().  A pesudo
(not compiled) code attached.

Thanks,

-- 
Peter Xu

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