Re: [PATCH v2 6/6] fork: Use __mt_dup() to duplicate maple tree in dup_mmap()

From: Peng Zhang
Date: Fri Sep 08 2023 - 06:01:08 EST



在 2023/9/8 04:14, Liam R. Howlett 写道:
* Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx> [230830 08:58]:
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. The optimization effect is proportional to the
number of VMAs.

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 numbers. There are 21 VMAs by default. The first row
indicates the number of added VMAs. The following two lines are the
number of fork() calls every 10 seconds. These numbers are different
from the test results in v1 because this time the benchmark is bound to
a CPU. This way the numbers are more stable.

Increment of VMAs: 0 100 200 400 800 1600 3200 6400
6.5.0-next-20230829: 111878 75531 53683 35282 20741 11317 6110 3158
Apply this patchset: 114531 85420 64541 44592 28660 16371 9038 4831
+2.37% +13.09% +20.23% +26.39% +38.18% +44.66% +47.92% +52.98%

Thanks!

Can you include 21 in this table since it's the default?
Maybe I didn't express clearly, "Increment of VMAs" means the number of
VMAs added on the basis of 21 VMAs.


[1] https://github.com/kdlucas/byte-unixbench/tree/master

Signed-off-by: Peng Zhang <zhangpeng.00@xxxxxxxxxxxxx>
---
kernel/fork.c | 34 ++++++++++++++++++++++++++--------
mm/mmap.c | 14 ++++++++++++--
2 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 3b6d20dfb9a8..e6299adefbd8 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 +677,39 @@ 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);

Apparently the flags should be GFP_KERNEL here so that compaction can
run.
OK, I'll change it to 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) {
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_store(&vmi.mas, XA_ZERO_ENTRY);
+ goto loop_out;
+ }
+

Storing NULL may need extra space as you noted, so we need to be careful
what happens if we don't have that space. We should have a testcase to
test this scenario.

mas_store_gfp() should be used with GFP_KERNEL. The VMAs use GFP_KERNEL
in this function, see vm_area_dup().

Don't use the exit_mmap() path to undo a failed fork. You've added
checks and complications to the exit path for all tasks in the very
unlikely event that we run out of memory when we hit a very unlikely
VM_DONTCOPY flag.

I see the issue with having a portion of the tree with new VMAs that are
accounted and a portion of the tree that has old VMAs that should not be
looked at. It was clever to use the XA_ZERO_ENTRY as a stop point, but
we cannot add that complication to the exit path and then there is the
OOM race to worry about (maybe, I am not sure since this MM isn't
active yet).

Using what is done in exit_mmap() and do_vmi_align_munmap() as a
prototype, we can do something like the *untested* code below:

if (unlikely(mas_is_err(&vmi.mas))) {
unsigned long max = vmi.index;

retval = xa_err(vmi.mas.node);
mas_set(&vmi.mas, 0);
tmp = mas_find(&vmi.mas, ULONG_MAX);
if (tmp) { /* Not the first VMA failed */
unsigned long nr_accounted = 0;

unmap_region(mm, &vmi.mas, vma, NULL, mpnt, 0, max, max,
true);
do {
if (vma->vm_flags & VM_ACCOUNT)
nr_accounted += vma_pages(vma);
remove_vma(vma, true);
cond_resched();
vma = mas_find(&vmi.mas, max - 1);
} while (vma != NULL);

vm_unacct_memory(nr_accounted);
}
__mt_destroy(&mm->mm_mt);
goto loop_out;
}

Once exit_mmap() is called, the check for OOM (no vma) will catch that
nothing is left to do.

It might be worth making an inline function to do this to keep the fork
code clean. We should test this by detecting a specific task name and
returning a failure at a given interval:

if (!strcmp(current->comm, "fork_test") {
...
}

Thank you for your suggestion, I will do this in the next version.


continue;
}
charge = 0;
@@ -750,8 +771,7 @@ 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_store(&vmi.mas, tmp);
mm->map_count++;
if (!(tmp->vm_flags & VM_WIPEONFORK))
@@ -778,8 +798,6 @@ 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 b56a7f0c9f85..dfc6881be81c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3196,7 +3196,11 @@ 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;
@@ -3234,7 +3238,13 @@ 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