Re: [PATCH v2 3/3] fork: lock VMAs of the parent process when forking

[Mateusz Guzik <mjguzik@gmail.com>]

On Sat, Jul 08, 2023 at 12:12:12PM -0700, Suren Baghdasaryan wrote:
[..]
> Lock VMAs of the parent process when forking a child, which prevents
> concurrent page faults during fork operation and avoids this issue.
> This fix can potentially regress some fork-heavy workloads. Kernel build
> time did not show noticeable regression on a 56-core machine while a
> stress test mapping 10000 VMAs and forking 5000 times in a tight loop
> shows ~5% regression. If such fork time regression is unacceptable,
> disabling CONFIG_PER_VMA_LOCK should restore its performance. Further
> optimizations are possible if this regression proves to be problematic.
> 
> ---
>  kernel/fork.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index b85814e614a5..d2e12b6d2b18 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -686,6 +686,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
>  	for_each_vma(old_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));
>  			continue;
> 

I don't see it mentioned in the discussion, so at a risk of ruffling
feathers or looking really bad I'm going to ask: is the locking of any
use if the forking process is single-threaded? The singular thread in
this case is occupied executing this very code, so it can't do any op
in parallel. Is there anyone else who could trigger a page fault? Are
these shared with other processes? Cursory reading suggests a private
copy is made here, so my guess is no. But then again, I landed here
freshly from the interwebz.

Or in short: if nobody can mess up the state if the forking process is
single-threaded, why not check for mm_users or whatever other indicator
to elide the slowdown for the (arguably) most common case?

If the state can be messed up anyway, that's a shame, but short
explanation how would be welcome.

to illustrate (totally untested):
diff --git a/kernel/fork.c b/kernel/fork.c
index d2e12b6d2b18..aac6b08a0b21 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -652,6 +652,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
        LIST_HEAD(uf);
        VMA_ITERATOR(old_vmi, oldmm, 0);
        VMA_ITERATOR(vmi, mm, 0);
+       bool singlethread = READ_ONCE(oldmm->mm_users) == 1;

        uprobe_start_dup_mmap();
        if (mmap_write_lock_killable(oldmm)) {
@@ -686,7 +687,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
        for_each_vma(old_vmi, mpnt) {
                struct file *file;

-               vma_start_write(mpnt);
+               if (!singelthreaded)
+                       vma_start_write(mpnt);
                if (mpnt->vm_flags & VM_DONTCOPY) {
                        vm_stat_account(mm, mpnt->vm_flags, -vma_pages(mpnt));
                        continue;