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Date:   Wed,  5 Jul 2023 18:13:59 -0700
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Subject: [PATCH v4 1/2] fork: lock VMAs of the parent process when forking
From:   Suren Baghdasaryan <surenb@google.com>
To:     akpm@linux-foundation.org
Cc:     jirislaby@kernel.org, jacobly.alt@gmail.com,
        holger@applied-asynchrony.com, hdegoede@redhat.com,
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        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Suren Baghdasaryan <surenb@google.com>
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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 ~7% 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@redhat.com>
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Closes: https://lore.kernel.org/all/dbdef34c-3a07-5951-e1ae-e9c6e3cdf51b@ke=
rnel.org/
Reported-by: Holger Hoffst=C3=A4tte <holger@applied-asynchrony.com>
Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@ap=
plied-asynchrony.com/
Reported-by: Jacob Young <jacobly.alt@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=3D217624
Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling first"=
)
Cc: stable@vger.kernel.org
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 kernel/fork.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index b85814e614a5..2ba918f83bde 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -658,6 +658,12 @@ static __latent_entropy int dup_mmap(struct mm_struct =
*mm,
 		retval =3D -EINTR;
 		goto fail_uprobe_end;
 	}
+#ifdef CONFIG_PER_VMA_LOCK
+	/* Disallow any page faults before calling flush_cache_dup_mm */
+	for_each_vma(old_vmi, mpnt)
+		vma_start_write(mpnt);
+	vma_iter_set(&old_vmi, 0);
+#endif
 	flush_cache_dup_mm(oldmm);
 	uprobe_dup_mmap(oldmm, mm);
 	/*
--=20
2.41.0.255.g8b1d071c50-goog