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

From: Peng Zhang
Date: Wed Oct 04 2023 - 05:11:04 EST



在 2023/10/4 02:46, Liam R. Howlett 写道:
* Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [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?

The following is a non-professional analysis of the algorithm.

Let's first analyze the average time complexity of the new algorithm, as
it is relatively easy to analyze. The maximum number of branches for
internal nodes in a maple tree in allocation mode is 10. However, to
simplify the analysis, we will not consider this case and assume that
all nodes have a maximum of 16 branches.

The new algorithm assumes that there is no case where a VMA with the
VM_DONTCOPY flag is deleted. If such a case exists, this analysis cannot
be applied.

The operations of the new algorithm consist of three parts:

1. DFS traversal of each node in the source tree
2. For each node in the source tree, create a copy and construct a new
node
3. Traverse the new tree using mas_find() and replace each element

If there are a total of n elements in the maple tree, we can conclude
that there are n/16 leaf nodes. Regarding the second-to-last level, we
can conclude that there are n/16^2 nodes. The total number of nodes in
the entire tree is given by the sum of n/16 + n/16^2 + n/16^3 + ... + 1.
This is a geometric progression with a total of log base 16 of n terms.
According to the formula for the sum of a geometric progression, the sum
is (n-1)/15. So, this tree has a total of (n-1)/15 nodes and
(n-1)/15 - 1 edges.

For the operations in the first part of this algorithm, since DFS
traverses each edge twice, the time complexity would be
2*((n-1)/15 - 1).

For the second part, each operation involves copying a node and making
necessary modifications. Therefore, the time complexity is
16*(n-1)/15.

For the third part, we use mas_find() to traverse and replace each
element, which is essentially similar to the combination of the first
and second parts. mas_find() traverses all nodes and within each node,
it iterates over all elements and performs replacements. The time
complexity of traversing the nodes is 2*((n-1)/15 - 1), and for all
nodes, the time complexity of replacing all their elements is
16*(n-1)/15.

By ignoring all constant factors, each of the three parts of the
algorithm has a time complexity of O(n). Therefore, this new algorithm
is O(n).

The exact time complexity of the old algorithm is difficult to analyze.
I can only provide an upper bound estimation. There are two possible
scenarios for each insertion:

1. Appending at the end of a node.
2. Splitting nodes multiple times.

For the first scenario, the individual operation has a time complexity
of O(1). As for the second scenario, it involves node splitting. The
challenge lies in determining which insertions trigger splits and how
many splits occur each time, which is difficult to calculate. In the
worst-case scenario, each insertion requires splitting the tree's height
log(n) times. Assuming every insertion is in the worst-case scenario,
the time complexity would be n*log(n). However, not every insertion
requires splitting, and the number of splits each time may not
necessarily be log(n). Therefore, this is an estimation of the upper
bound.


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@xxxxxxxxxxxxx>
---
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().
Unfortunately, it cannot be done this way. I fell into this trap before,
and it caused incomplete page table cleanup. To solve this problem, the
only solution I can think of right now is to add an additional
parameter.

free_pgtables() will be called in unmap_region() to free the page table,
like this:

free_pgtables(&tlb, mas, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
next ? next->vm_start : USER_PGTABLES_CEILING,
mm_wr_locked);

The problem is with 'next'. Our 'vma_end' does not exist in the actual
mmap because it has not been duplicated and cannot be used as 'next'.
If there is a real 'next', we can use 'next->vm_start' as the ceiling,
which is not a problem. If there is no 'next' (next is 'vma_end'), we
can only use 'USER_PGTABLES_CEILING' as the ceiling. Using
'vma_end->vm_start' as the ceiling will cause the page table not to be
fully freed, which may be related to alignment in 'free_pgd_range()'. To
solve this problem, we have to introduce 'tree_end', and separating
'tree_end' and 'ceiling' can solve this problem.


+
+ 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