Re: [PATCH v5 10/10] fork: Use __mt_dup() to duplicate maple tree in dup_mmap()

[Liam R. Howlett]

* Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [231015 23:23]:
> 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 efficient.
> 
> The average time complexity of __mt_dup() is O(n), where n is the number
> of VMAs. The proof of the time complexity is provided in the commit log
> that introduces __mt_dup(). After duplicating the maple tree, each element
> is traversed and replaced (ignoring the cases of deletion, which are rare).
> Since it is only a replacement operation for each element, this process is
> also O(n).
> 
> Analyzing the exact time complexity of the previous algorithm is
> challenging because each insertion can involve appending to a node, pushing
> data to adjacent nodes, or even splitting nodes. The frequency of each
> action is difficult to calculate. The worst-case scenario for a single
> insertion is when the tree undergoes splitting at every level. If we
> consider each insertion as the worst-case scenario, we can determine that
> the upper bound of the time complexity is O(n*log(n)), although this is a
> loose upper bound. However, based on the test data, it appears that the
> actual time complexity is likely to be O(n).
> 
> 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. To handle this, we mark the failure point with
> XA_ZERO_ENTRY. In exit_mmap(), if this marker is encountered, stop
> releasing VMAs that have not been duplicated after this point.
> 
> 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. The first row shows the number of VMAs.
> The second and third rows show the number of fork() calls per ten seconds,
> corresponding to next-20231006 and the this patchset, respectively. 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.
> 
> 21     121   221    421    821    1621   3221   6421   12821  25621  51221
> 112100 76261 54227  34035  20195  11112  6017   3161   1606   802    393
> 114558 83067 65008  45824  28751  16072  8922   4747   2436   1233   599
> 2.19%  8.92% 19.88% 34.64% 42.37% 44.64% 48.28% 50.17% 51.68% 53.74% 52.42%
> 
> [1] https://github.com/kdlucas/byte-unixbench/tree/master
> 
> Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
> ---
>  kernel/fork.c | 39 ++++++++++++++++++++++++++++-----------
>  mm/memory.c   |  7 ++++++-
>  mm/mmap.c     |  9 ++++++---
>  3 files changed, 40 insertions(+), 15 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 0ff2e0cd4109..0be15501e52e 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 +677,21 @@ 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) {
> +			retval = mas_store_gfp(&vmi.mas, NULL, GFP_KERNEL);

vma_iter_clear_gfp() exists, but needs to be relocated from internal.h
to mm.h to be used here.

> +			if (retval)
> +				goto loop_out;
> +
>  			vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
>  			continue;
>  		}
> @@ -749,9 +753,11 @@ 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.
> +		 */

Allocations didn't happen here with the bulk store either, and the
vma_iter_bulk_store() does a mas_store() - see include/linux/mm.h

The vma iterator is used when possible for type safety.

> +		mas_store(&vmi.mas, tmp);
>  
>  		mm->map_count++;
>  		if (!(tmp->vm_flags & VM_WIPEONFORK))
> @@ -760,15 +766,28 @@ 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 (!retval) {
>  		mt_set_in_rcu(vmi.mas.tree);
> +	} else if (mpnt) {
> +		/*
> +		 * The entire maple tree has already been duplicated. If the
> +		 * mmap duplication fails, mark the failure point with
> +		 * XA_ZERO_ENTRY. In exit_mmap(), if this marker is encountered,
> +		 * stop releasing VMAs that have not been duplicated after this
> +		 * point.
> +		 */
> +		mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
> +		mas_store(&vmi.mas, XA_ZERO_ENTRY);

vma_iter_clear() exists but uses the preallocation call.  I really don't
want mas_ calls where unnecessary, but this seems like a special case.
We have a vma iterator here so it's messy.

> +	}
>  out:
>  	mmap_write_unlock(mm);
>  	flush_tlb_mm(oldmm);
> @@ -778,8 +797,6 @@ 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/memory.c b/mm/memory.c
> index b320af6466cc..ea48bd4b1feb 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -374,6 +374,8 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
>  		 * be 0.  This will underflow and is okay.
>  		 */
>  		next = mas_find(mas, ceiling - 1);
> +		if (unlikely(xa_is_zero(next)))
> +			next = NULL;
>  
>  		/*
>  		 * Hide vma from rmap and truncate_pagecache before freeing
> @@ -395,6 +397,8 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
>  			       && !is_vm_hugetlb_page(next)) {
>  				vma = next;
>  				next = mas_find(mas, ceiling - 1);
> +				if (unlikely(xa_is_zero(next)))
> +					next = NULL;
>  				if (mm_wr_locked)
>  					vma_start_write(vma);
>  				unlink_anon_vmas(vma);
> @@ -1743,7 +1747,8 @@ void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas,
>  		unmap_single_vma(tlb, vma, start, end, &details,
>  				 mm_wr_locked);
>  		hugetlb_zap_end(vma, &details);
> -	} while ((vma = mas_find(mas, tree_end - 1)) != NULL);
> +		vma = mas_find(mas, tree_end - 1);
> +	} while (vma && likely(!xa_is_zero(vma)));
>  	mmu_notifier_invalidate_range_end(&range);
>  }
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 1855a2d84200..12ce17863e62 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3213,10 +3213,11 @@ void exit_mmap(struct mm_struct *mm)
>  	arch_exit_mmap(mm);
>  
>  	vma = mas_find(&mas, ULONG_MAX);
> -	if (!vma) {
> +	if (!vma || unlikely(xa_is_zero(vma))) {
>  		/* Can happen if dup_mmap() received an OOM */
>  		mmap_read_unlock(mm);
> -		return;
> +		mmap_write_lock(mm);
> +		goto destroy;
>  	}
>  
>  	lru_add_drain();
> @@ -3251,11 +3252,13 @@ void exit_mmap(struct mm_struct *mm)
>  		remove_vma(vma, true);
>  		count++;
>  		cond_resched();
> -	} while ((vma = mas_find(&mas, ULONG_MAX)) != NULL);
> +		vma = mas_find(&mas, ULONG_MAX);
> +	} while (vma && likely(!xa_is_zero(vma)));
>  
>  	BUG_ON(count != mm->map_count);
>  
>  	trace_exit_mmap(mm);
> +destroy:
>  	__mt_destroy(&mm->mm_mt);
>  	mmap_write_unlock(mm);
>  	vm_unacct_memory(nr_accounted);
> -- 
> 2.20.1
>