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Message-ID: <49f0181a-55a4-41aa-8596-877560c8b802@bytedance.com>
Date: Mon, 9 Oct 2023 19:06:05 +0800
From: Peng Zhang <zhangpeng.00@...edance.com>
To: Liam.Howlett@...cle.com, corbet@....net, akpm@...ux-foundation.org,
willy@...radead.org, brauner@...nel.org, surenb@...gle.com,
michael.christie@...cle.com, mjguzik@...il.com,
mathieu.desnoyers@...icios.com, npiggin@...il.com,
peterz@...radead.org, oliver.sang@...el.com, mst@...hat.com
Cc: maple-tree@...ts.infradead.org, linux-mm@...ck.org,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Peng Zhang <zhangpeng.00@...edance.com>
Subject: Re: [PATCH v4 10/10] fork: Use __mt_dup() to duplicate maple tree in
dup_mmap()
在 2023/10/9 17:03, Peng Zhang 写道:
> 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. 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. 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%
There is still some room for optimization here. The test data after replacing
'mas_store()' with 'mas_replace_entry()' during the process of replacing VMA
is as follows:
112100 76261 54227 34035 20195 11112 6017 3161 1606 802 393
112517 86919 67019 46338 30194 17554 9788 5250 2729 1393 680
0.37% 13.98% 23.59% 36.15% 49.51% 57.97% 62.67% 66.09% 69.93% 73.69% 73.03%
But we have discussed many times before and haven't come up with the best way
to do it. It may be necessary to introduce a write type for maple tree, but
I haven't figured out all the details, so I don't plan to do it for now.
mas_replace_entry():
void mas_replace_entry(struct ma_state *mas, void *entry)
{
void __rcu **slots;
slots = ma_slots(mte_to_node(mas->node), mte_node_type(mas->node));
rcu_assign_pointer(slots[mas->offset], entry);
}
>
> [1] https://github.com/kdlucas/byte-unixbench/tree/master
>
> Signed-off-by: Peng Zhang <zhangpeng.00@...edance.com>
> ---
> include/linux/mm.h | 1 +
> kernel/fork.c | 34 +++++++++++++++++++++----------
> mm/internal.h | 3 ++-
> mm/memory.c | 7 ++++---
> mm/mmap.c | 50 ++++++++++++++++++++++++++++++++++++++++++++--
> 5 files changed, 78 insertions(+), 17 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 14e50925b76d..d039f10d258e 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3248,6 +3248,7 @@ 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 0ff2e0cd4109..5f24f6d68ea4 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 18e360fa53bc..bcd92a5b5474 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 b320af6466cc..51bb1d16a54e 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -363,7 +363,8 @@ 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;
> @@ -373,7 +374,7 @@ 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
> @@ -394,7 +395,7 @@ 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 1855a2d84200..d044d68d1361 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2337,7 +2337,7 @@ 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);
> }
>
> @@ -3197,6 +3197,52 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
> }
> EXPORT_SYMBOL(vm_brk_flags);
>
> +void undo_dup_mmap(struct mm_struct *mm, struct vm_area_struct *vma_end)
> +{
> + unsigned long tree_end = USER_PGTABLES_CEILING;
> + 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 USER_PGTABLES_CEILING will
> + * unmap all VMAs in the maple tree.
> + */
> + if (vma_end) {
> + tree_end = vma_end->vm_start;
> + if (tree_end == 0)
> + goto destroy;
> + }
> +
> + vma = vma_find(&vmi, tree_end);
> + if (!vma)
> + goto destroy;
> +
> + arch_unmap(mm, vma->vm_start, tree_end);
> +
> + vma_iter_set(&vmi, vma->vm_end);
> + unmap_region(mm, &vmi.mas, vma, NULL, NULL, 0, tree_end, tree_end, true);
> +
> + vma_iter_set(&vmi, vma->vm_end);
> + do {
> + if (vma->vm_flags & VM_ACCOUNT)
> + nr_accounted += vma_pages(vma);
> + remove_vma(vma, true);
> + count++;
> + cond_resched();
> + } for_each_vma_range(vmi, vma, tree_end);
> +
> + 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)
> {
> @@ -3236,7 +3282,7 @@ 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);
>
> /*
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