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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,
michel@lespinasse.org, jglisse@google.com, mhocko@suse.com,
vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net,
dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com,
peterz@infradead.org, ldufour@linux.ibm.com, paulmck@kernel.org,
mingo@redhat.com, will@kernel.org, luto@kernel.org,
songliubraving@fb.com, peterx@redhat.com, david@redhat.com,
dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de,
kent.overstreet@linux.dev, punit.agrawal@bytedance.com,
lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com,
chriscli@google.com, axelrasmussen@google.com, joelaf@google.com,
minchan@google.com, rppt@kernel.org, jannh@google.com,
shakeelb@google.com, tatashin@google.com, edumazet@google.com,
gthelen@google.com, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, stable@vger.kernel.org,
Suren Baghdasaryan <surenb@google.com>
Subject: [PATCH v4 1/2] fork: lock VMAs of the parent process when forking
Date: Wed, 5 Jul 2023 18:13:59 -0700 [thread overview]
Message-ID: <20230706011400.2949242-2-surenb@google.com> (raw)
In-Reply-To: <20230706011400.2949242-1-surenb@google.com>
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@kernel.org/
Reported-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Closes: https://lore.kernel.org/all/b198d649-f4bf-b971-31d0-e8433ec2a34c@applied-asynchrony.com/
Reported-by: Jacob Young <jacobly.alt@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@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 = -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);
/*
--
2.41.0.255.g8b1d071c50-goog
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