Re: [PATCH v2 1/2] fork: lock VMAs of the parent process when forking

[David Hildenbrand]

On 05.07.23 08:37, Suren Baghdasaryan wrote:
> When forking a child process, parent write-protects an anonymous page
> and COW-shares it with the child being forked using copy_present_pte().
> Parent's TLB is flushed right before we drop the parent's mmap_lock in
> dup_mmap(). If we get a write-fault before that TLB flush in the parent,
> and we end up replacing that anonymous page in the parent process in
> do_wp_page() (because, COW-shared with the child), this might lead to
> some stale writable TLB entries targeting the wrong (old) page.
> Similar issue happened in the past with userfaultfd (see flush_tlb_page()
> call inside do_wp_page()).
> 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.

Out of interest, did you also populate page tables / pages for some of these
VMAs, or is this primarily looping over 10000 VMAs that don't actually copy any
page tables?

> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Reported-by: Jiri Slaby <jirislaby@xxxxxxxxxx>
> Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@xxxxxxxxxx/
> Reported-by: Holger Hoffstätte <holger@xxxxxxxxxxxxxxxxxxxxxx>
> Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@xxxxxxxxxxxxxxxxxxxxxx/
> Reported-by: Jacob Young <jacobly.alt@xxxxxxxxx>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624
> Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> ---
>   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;

After the mmap_write_lock_killable(), there will still be a period where page
faults can happen. Essentially, page faults can happen for a VMA until we lock that VMA.

I cannot immediately name something that is broken allowing for that, and this change
should fix the issue at hand, but exotic things like

	flush_cache_dup_mm(oldmm);

make me wonder if we really want to allow for that or if there is some other corner case
in fork() handling that really doesn't expect concurrent page faults (and, thereby, page
table modifications) with fork.

For example, documentation/core-api/cachetlb.rst says

2) ``void flush_cache_dup_mm(struct mm_struct *mm)``

	This interface flushes an entire user address space from
	the caches.  That is, after running, there will be no cache
	lines associated with 'mm'.

	This interface is used to handle whole address space
	page table operations such as what happens during fork.

	This option is separate from flush_cache_mm to allow some
	optimizations for VIPT caches.


An alternative that requires another VMA walk would be

diff --git a/kernel/fork.c b/kernel/fork.c
index 41c964104b58..0f182d3f049b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -662,6 +662,13 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 		retval = -EINTR;
 		goto fail_uprobe_end;
 	}
+
+	/* Disallow any page faults early by locking all VMAs. */
+	if (IS_ENABLED(CONFIG_PER_VMA_LOCK)) {
+		for_each_vma(old_vmi, mpnt)
+			vma_start_write(mpnt);
+		vma_iter_init(old_vmi, old_mm, 0);
+	}
 	flush_cache_dup_mm(oldmm);
 	uprobe_dup_mmap(oldmm, mm);
 	/*
--
2.41.0


Unless there are other thoughts, I guess you change is fine regarding the problem
at hand. Not so sure regarding any other corner cases, that's why I'm spelling it out.


Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers,

David / dhildenb