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Date:   Wed, 5 Jul 2023 19:14:05 +0200
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Subject: Re: [PATCH v3 1/2] fork: lock VMAs of the parent process when forking
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To:     Suren Baghdasaryan <surenb@google.com>, akpm@linux-foundation.org
Cc:     jirislaby@kernel.org, jacobly.alt@gmail.com,
        holger@applied-asynchrony.com, hdegoede@redhat.com,
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References: <20230705171213.2843068-1-surenb@google.com>
 <20230705171213.2843068-2-surenb@google.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230705171213.2843068-2-surenb@google.com>
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On 05.07.23 19:12, 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.
> 
> 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..403bc2b72301 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_init(&old_vmi, oldmm, 0);
> +#endif
>   	flush_cache_dup_mm(oldmm);
>   	uprobe_dup_mmap(oldmm, mm);
>   	/*

The old version was most probably fine as well, but this certainly looks 
even safer.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb