Re: [PATCH v3 9/9] fork: Use __mt_dup() to duplicate maple tree in dup_mmap()

["Liam R. Howlett" <Liam.Howlett@Oracle.com>]

* Peng Zhang <zhangpeng.00@bytedance.com> [230924 23:58]:
> In dup_mmap(), using __mt_dup() to duplicate the old maple tree and then
> directly replacing the entries of VMAs in the new maple tree can result
> in better performance. __mt_dup() uses DFS pre-order to duplicate the
> maple tree, so it is very efficient. The average time complexity of
> duplicating VMAs is reduced from O(n * log(n)) to O(n). The optimization
> effect is proportional to the number of VMAs.

I am not confident in the big O calculations here.  Although the addition
of the tree is reduced, adding a VMA still needs to create the nodes
above it - which are a function of n.  How did you get O(n * log(n)) for
the existing fork?

I would think your new algorithm is n * log(n/16), while the
previous was n * log(n/16) * f(n).  Where f(n) would be something
to do with the decision to split/rebalance in bulk insert mode.

It's certainly a better algorithm to duplicate trees, but I don't think
it is O(n).  Can you please explain?

> 
> As the entire maple tree is duplicated using __mt_dup(), if dup_mmap()
> fails, there will be a portion of VMAs that have not been duplicated in
> the maple tree. This makes it impossible to unmap all VMAs in exit_mmap().
> To solve this problem, undo_dup_mmap() is introduced to handle the failure
> of dup_mmap(). I have carefully tested the failure path and so far it
> seems there are no issues.
> 
> There is a "spawn" in byte-unixbench[1], which can be used to test the
> performance of fork(). I modified it slightly to make it work with
> different number of VMAs.
> 
> Below are the test results. By default, there are 21 VMAs. The first row
> shows the number of additional VMAs added on top of the default. The last
> two rows show the number of fork() calls per ten seconds. The test results
> were obtained with CPU binding to avoid scheduler load balancing that
> could cause unstable results. There are still some fluctuations in the
> test results, but at least they are better than the original performance.
> 
> Increment of VMAs: 0      100     200     400     800     1600    3200    6400
> next-20230921:     112326 75469   54529   34619   20750   11355   6115    3183
> Apply this:        116505 85971   67121   46080   29722   16665   9050    4805
>                    +3.72% +13.92% +23.09% +33.11% +43.24% +46.76% +48.00% +50.96%
> 
> [1] https://github.com/kdlucas/byte-unixbench/tree/master
> 
> Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
> ---
>  include/linux/mm.h |  1 +
>  kernel/fork.c      | 34 ++++++++++++++++++++----------
>  mm/internal.h      |  3 ++-
>  mm/memory.c        |  7 ++++---
>  mm/mmap.c          | 52 ++++++++++++++++++++++++++++++++++++++++++++--
>  5 files changed, 80 insertions(+), 17 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 1f1d0d6b8f20..10c59dc7ffaa 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3242,6 +3242,7 @@ extern void unlink_file_vma(struct vm_area_struct *);
>  extern struct vm_area_struct *copy_vma(struct vm_area_struct **,
>  	unsigned long addr, unsigned long len, pgoff_t pgoff,
>  	bool *need_rmap_locks);
> +extern void undo_dup_mmap(struct mm_struct *mm, struct vm_area_struct *vma_end);
>  extern void exit_mmap(struct mm_struct *);
>  
>  static inline int check_data_rlimit(unsigned long rlim,
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 7ae36c2e7290..2f3d83e89fe6 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -650,7 +650,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  	int retval;
>  	unsigned long charge = 0;
>  	LIST_HEAD(uf);
> -	VMA_ITERATOR(old_vmi, oldmm, 0);
>  	VMA_ITERATOR(vmi, mm, 0);
>  
>  	uprobe_start_dup_mmap();
> @@ -678,16 +677,25 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  		goto out;
>  	khugepaged_fork(mm, oldmm);
>  
> -	retval = vma_iter_bulk_alloc(&vmi, oldmm->map_count);
> -	if (retval)
> +	/* Use __mt_dup() to efficiently build an identical maple tree. */
> +	retval = __mt_dup(&oldmm->mm_mt, &mm->mm_mt, GFP_KERNEL);
> +	if (unlikely(retval))
>  		goto out;
>  
>  	mt_clear_in_rcu(vmi.mas.tree);
> -	for_each_vma(old_vmi, mpnt) {
> +	for_each_vma(vmi, mpnt) {
>  		struct file *file;
>  
>  		vma_start_write(mpnt);
>  		if (mpnt->vm_flags & VM_DONTCOPY) {
> +			mas_store_gfp(&vmi.mas, NULL, GFP_KERNEL);
> +
> +			/* If failed, undo all completed duplications. */
> +			if (unlikely(mas_is_err(&vmi.mas))) {
> +				retval = xa_err(vmi.mas.node);
> +				goto loop_out;
> +			}
> +
>  			vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
>  			continue;
>  		}
> @@ -749,9 +757,11 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  		if (is_vm_hugetlb_page(tmp))
>  			hugetlb_dup_vma_private(tmp);
>  
> -		/* Link the vma into the MT */
> -		if (vma_iter_bulk_store(&vmi, tmp))
> -			goto fail_nomem_vmi_store;
> +		/*
> +		 * Link the vma into the MT. After using __mt_dup(), memory
> +		 * allocation is not necessary here, so it cannot fail.
> +		 */
> +		mas_store(&vmi.mas, tmp);
>  
>  		mm->map_count++;
>  		if (!(tmp->vm_flags & VM_WIPEONFORK))
> @@ -760,15 +770,19 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  		if (tmp->vm_ops && tmp->vm_ops->open)
>  			tmp->vm_ops->open(tmp);
>  
> -		if (retval)
> +		if (retval) {
> +			mpnt = vma_next(&vmi);
>  			goto loop_out;
> +		}
>  	}
>  	/* a new mm has just been created */
>  	retval = arch_dup_mmap(oldmm, mm);
>  loop_out:
>  	vma_iter_free(&vmi);
> -	if (!retval)
> +	if (likely(!retval))
>  		mt_set_in_rcu(vmi.mas.tree);
> +	else
> +		undo_dup_mmap(mm, mpnt);
>  out:
>  	mmap_write_unlock(mm);
>  	flush_tlb_mm(oldmm);
> @@ -778,8 +792,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  	uprobe_end_dup_mmap();
>  	return retval;
>  
> -fail_nomem_vmi_store:
> -	unlink_anon_vmas(tmp);
>  fail_nomem_anon_vma_fork:
>  	mpol_put(vma_policy(tmp));
>  fail_nomem_policy:
> diff --git a/mm/internal.h b/mm/internal.h
> index 7a961d12b088..288ec81770cb 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -111,7 +111,8 @@ void folio_activate(struct folio *folio);
>  
>  void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
>  		   struct vm_area_struct *start_vma, unsigned long floor,
> -		   unsigned long ceiling, bool mm_wr_locked);
> +		   unsigned long ceiling, unsigned long tree_end,
> +		   bool mm_wr_locked);
>  void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte);
>  
>  struct zap_details;
> diff --git a/mm/memory.c b/mm/memory.c
> index 983a40f8ee62..1fd66a0d5838 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -362,7 +362,8 @@ void free_pgd_range(struct mmu_gather *tlb,
>  
>  void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
>  		   struct vm_area_struct *vma, unsigned long floor,
> -		   unsigned long ceiling, bool mm_wr_locked)
> +		   unsigned long ceiling, unsigned long tree_end,
> +		   bool mm_wr_locked)
>  {
>  	do {
>  		unsigned long addr = vma->vm_start;
> @@ -372,7 +373,7 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
>  		 * Note: USER_PGTABLES_CEILING may be passed as ceiling and may
>  		 * be 0.  This will underflow and is okay.
>  		 */
> -		next = mas_find(mas, ceiling - 1);
> +		next = mas_find(mas, tree_end - 1);
>  
>  		/*
>  		 * Hide vma from rmap and truncate_pagecache before freeing
> @@ -393,7 +394,7 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
>  			while (next && next->vm_start <= vma->vm_end + PMD_SIZE
>  			       && !is_vm_hugetlb_page(next)) {
>  				vma = next;
> -				next = mas_find(mas, ceiling - 1);
> +				next = mas_find(mas, tree_end - 1);
>  				if (mm_wr_locked)
>  					vma_start_write(vma);
>  				unlink_anon_vmas(vma);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ad950f773e4..daed3b423124 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2312,7 +2312,7 @@ static void unmap_region(struct mm_struct *mm, struct ma_state *mas,
>  	mas_set(mas, mt_start);
>  	free_pgtables(&tlb, mas, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
>  				 next ? next->vm_start : USER_PGTABLES_CEILING,
> -				 mm_wr_locked);
> +				 tree_end, mm_wr_locked);
>  	tlb_finish_mmu(&tlb);
>  }
>  
> @@ -3178,6 +3178,54 @@ int vm_brk(unsigned long addr, unsigned long len)
>  }
>  EXPORT_SYMBOL(vm_brk);
>  
> +void undo_dup_mmap(struct mm_struct *mm, struct vm_area_struct *vma_end)
> +{
> +	unsigned long tree_end;
> +	VMA_ITERATOR(vmi, mm, 0);
> +	struct vm_area_struct *vma;
> +	unsigned long nr_accounted = 0;
> +	int count = 0;
> +
> +	/*
> +	 * vma_end points to the first VMA that has not been duplicated. We need
> +	 * to unmap all VMAs before it.
> +	 * If vma_end is NULL, it means that all VMAs in the maple tree have
> +	 * been duplicated, so setting tree_end to 0 will overflow to ULONG_MAX
> +	 * when using it.
> +	 */
> +	if (vma_end) {
> +		tree_end = vma_end->vm_start;
> +		if (tree_end == 0)
> +			goto destroy;
> +	} else
> +		tree_end = 0;
> +
> +	vma = mas_find(&vmi.mas, tree_end - 1);
> +
> +	if (vma) {
> +		arch_unmap(mm, vma->vm_start, tree_end);
> +		unmap_region(mm, &vmi.mas, vma, NULL, NULL, 0, tree_end,
> +			     tree_end, true);

next is vma_end, as per your comment above.  Using next = vma_end allows
you to avoid adding another argument to free_pgtables().

> +
> +		mas_set(&vmi.mas, vma->vm_end);
> +		do {
> +			if (vma->vm_flags & VM_ACCOUNT)
> +				nr_accounted += vma_pages(vma);
> +			remove_vma(vma, true);
> +			count++;
> +			cond_resched();
> +			vma = mas_find(&vmi.mas, tree_end - 1);
> +		} while (vma != NULL);
> +
> +		BUG_ON(count != mm->map_count);
> +
> +		vm_unacct_memory(nr_accounted);
> +	}
> +
> +destroy:
> +	__mt_destroy(&mm->mm_mt);
> +}
> +
>  /* Release all mmaps. */
>  void exit_mmap(struct mm_struct *mm)
>  {
> @@ -3217,7 +3265,7 @@ void exit_mmap(struct mm_struct *mm)
>  	mt_clear_in_rcu(&mm->mm_mt);
>  	mas_set(&mas, vma->vm_end);
>  	free_pgtables(&tlb, &mas, vma, FIRST_USER_ADDRESS,
> -		      USER_PGTABLES_CEILING, true);
> +		      USER_PGTABLES_CEILING, USER_PGTABLES_CEILING, true);
>  	tlb_finish_mmu(&tlb);
>  
>  	/*
> -- 
> 2.20.1
>