Use __mt_dup() to duplicate the old maple tree in dup_mmap(), and then
directly modify the entries of VMAs in the new maple tree, which can
get better performance. dup_mmap() is used by fork(), so this patch
optimizes fork(). The optimization effect is proportional to the number
of VMAs.
Due to the introduction of this method, the optimization in
(maple_tree: add a fast path case in mas_wr_slot_store())[1] no longer
has an effect here, but it is also an optimization of the maple tree.
There is a unixbench test suite[2] where 'spawn' is used to test fork().
'spawn' only has 23 VMAs by default, so I tweaked the benchmark code a
bit to use mmap() to control the number of VMAs. Therefore, the
performance under different numbers of VMAs can be measured.
Insert code like below into 'spawn':
for (int i = 0; i < 200; ++i) {
size_t size = 10 * getpagesize();
void *addr;
if (i & 1) {
addr = mmap(NULL, size, PROT_READ,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
} else {
addr = mmap(NULL, size, PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
}
if (addr == MAP_FAILED)
...
}
Based on next-20230721, use 'spawn' under 23, 203, and 4023 VMAs, test
4 times in 30 seconds each time, and get the following numbers. These
numbers are the number of fork() successes in 30s (average of the best
3 out of 4). By the way, based on next-20230725, I reverted [1], and
tested it together as a comparison. In order to ensure the reliability
of the test results, these tests were run on a physical machine.
23VMAs 223VMAs 4023VMAs
revert [1]: 159104.00 73316.33 6787.00
+0.77% +0.42% +0.28%
next-20230721: 160321.67 73624.67 6806.33
+2.77% +15.42% +29.86%
apply this: 164751.67 84980.33 8838.67
It can be seen that the performance improvement is proportional to
the number of VMAs. With 23 VMAs, performance improves by about 3%,
with 223 VMAs, performance improves by about 15%, and with 4023 VMAs,
performance improves by about 30%.
[1] https://lore.kernel.org/lkml/20230628073657.75314-4-zhangpeng.00@xxxxxxxxxxxxx/
[2] https://github.com/kdlucas/byte-unixbench/tree/master
Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
---
kernel/fork.c | 35 +++++++++++++++++++++++++++--------
mm/mmap.c | 14 ++++++++++++--
2 files changed, 39 insertions(+), 10 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index f81149739eb9..ef80025b62d6 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();
@@ -678,17 +677,40 @@ 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_NOWAIT | __GFP_NOWARN);
+ 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) {
vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
+
+ /*
+ * Since the new tree is exactly the same as the old one,
+ * we need to remove the unneeded VMAs.
+ */
+ mas_store(&vmi.mas, NULL);
+
+ /*
+ * Even removing an entry may require memory allocation,
+ * and if removal fails, we use XA_ZERO_ENTRY to mark
+ * from which VMA it failed. The case of encountering
+ * XA_ZERO_ENTRY will be handled in exit_mmap().
+ */
+ if (unlikely(mas_is_err(&vmi.mas))) {
+ retval = xa_err(vmi.mas.node);
+ mas_reset(&vmi.mas);
+ if (mas_find(&vmi.mas, ULONG_MAX))
+ mas_replace_entry(&vmi.mas,
+ XA_ZERO_ENTRY);
+ goto loop_out;
+ }
+
continue;
}
charge = 0;
@@ -750,8 +772,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
hugetlb_dup_vma_private(tmp);
/* Link the vma into the MT */
- if (vma_iter_bulk_store(&vmi, tmp))
- goto fail_nomem_vmi_store;
+ mas_replace_entry(&vmi.mas, tmp);
mm->map_count++;
if (!(tmp->vm_flags & VM_WIPEONFORK))
@@ -778,8 +799,6 @@ 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/mmap.c b/mm/mmap.c
index bc91d91261ab..5bfba2fb0e39 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3184,7 +3184,11 @@ void exit_mmap(struct mm_struct *mm)
arch_exit_mmap(mm);
vma = mas_find(&mas, ULONG_MAX);
- if (!vma) {
+ /*
+ * If dup_mmap() fails to remove a VMA marked VM_DONTCOPY,
+ * xa_is_zero(vma) may be true.
+ */
+ if (!vma || xa_is_zero(vma)) {
/* Can happen if dup_mmap() received an OOM */
mmap_read_unlock(mm);
return;
@@ -3222,7 +3226,13 @@ 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);
+ /*
+ * If xa_is_zero(vma) is true, it means that subsequent VMAs
+ * donot need to be removed. Can happen if dup_mmap() fails to
+ * remove a VMA marked VM_DONTCOPY.
+ */
+ } while (vma != NULL && !xa_is_zero(vma));
BUG_ON(count != mm->map_count);
--
2.20.1
