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

[Suren Baghdasaryan]

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.

Suggested-by: David Hildenbrand <david at redhat.com>
Reported-by: Jiri Slaby <jirislaby at kernel.org>
Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@kernel.org/
Reported-by: Holger Hoffstätte <holger at applied-asynchrony.com>
Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/
Reported-by: Jacob Young <jacobly.alt at gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217624
Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first")
Cc: stable at vger.kernel.org
Signed-off-by: Suren Baghdasaryan <surenb at google.com>
---
 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;
-- 
2.41.0.390.g38632f3daf-goog