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Date:   Thu, 6 Jul 2023 09:18:53 -0400
From:   "Liam R. Howlett" <Liam.Howlett@Oracle.com>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     akpm@linux-foundation.org, jirislaby@kernel.org,
        jacobly.alt@gmail.com, holger@applied-asynchrony.com,
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        tatashin@google.com, edumazet@google.com, gthelen@google.com,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
Subject: Re: [PATCH v4 1/2] fork: lock VMAs of the parent process when forking
Message-ID: <20230706131853.cqnxdzbltnsaxxo4@revolver>
Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
        Suren Baghdasaryan <surenb@google.com>, akpm@linux-foundation.org,
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        vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net,
        dave@stgolabs.net, willy@infradead.org, peterz@infradead.org,
        ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com,
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        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
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References: <20230706011400.2949242-1-surenb@google.com>
 <20230706011400.2949242-2-surenb@google.com>
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* Suren Baghdasaryan <surenb@google.com> [230705 21:14]:
> 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.
>=20
> 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 Hoffst=E4tte <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=3D217624
> Fixes: 0bff0aaea03e ("x86/mm: try VMA lock-based page fault handling firs=
t")
> Cc: stable@vger.kernel.org
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>

> ---
>  kernel/fork.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>=20
> 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_struc=
t *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
>=20