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Date:   Fri, 7 Jul 2023 12:27:50 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     willy@infradead.org, liam.howlett@oracle.com, david@redhat.com,
        peterx@redhat.com, vbabka@suse.cz, michel@lespinasse.org,
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        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 1/2] mm: lock a vma before stack expansion
Message-Id: <20230707122750.f7cd77fe19b625cff37423ed@linux-foundation.org>
In-Reply-To: <20230707043211.3682710-1-surenb@google.com>
References: <20230707043211.3682710-1-surenb@google.com>
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On Thu,  6 Jul 2023 21:32:10 -0700 Suren Baghdasaryan <surenb@google.com> wrote:

> With recent changes necessitating mmap_lock to be held for write while
> expanding a stack, per-VMA locks should follow the same rules and be
> write-locked to prevent page faults into the VMA being expanded. Add
> the necessary locking.

What are the possible runtime effects of this change?

> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1977,6 +1977,8 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
>  		return -ENOMEM;
>  	}
>  
> +	/* Lock the VMA before expanding to prevent concurrent page faults */
> +	vma_start_write(vma);
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
>  	 * is required to hold the mmap_lock in read mode.  We need the
> @@ -2064,6 +2066,8 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
>  		return -ENOMEM;
>  	}
>  
> +	/* Lock the VMA before expanding to prevent concurrent page faults */
> +	vma_start_write(vma);
>  	/*
>  	 * vma->vm_start/vm_end cannot change under us because the caller
>  	 * is required to hold the mmap_lock in read mode.  We need the
> -- 
> 2.41.0.255.g8b1d071c50-goog