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Date:   Sun, 1 Aug 2021 12:33:35 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Luigi Rizzo <lrizzo@google.com>
Cc:     linux-kernel@vger.kernel.org, David Rientjes <rientjes@google.com>,
        linux-mm@kvack.org
Subject: Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()
Message-Id: <20210801123335.6a7f8e1ee1e52ea64db80323@linux-foundation.org>
In-Reply-To: <20210731175341.3458608-1-lrizzo@google.com>
References: <20210731175341.3458608-1-lrizzo@google.com>
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On Sat, 31 Jul 2021 10:53:41 -0700 Luigi Rizzo <lrizzo@google.com> wrote:

> find_vma() and variants need protection when used.
> This patch adds mmap_assert_lock() calls in the functions.
> 
> To make sure the invariant is satisfied, we also need to add a
> mmap_read_loc() around the get_user_pages_remote() call in
> get_arg_page(). The lock is not strictly necessary because the mm
> has been newly created, but the extra cost is limited because
> the same mutex was also acquired shortly before in __bprm_mm_init(),
> so it is hot and uncontended.
> 

Well, it isn't cost-free.  find_vma() is called a lot and a surprising
number of systems apparently run with CONFIG_DEBUG_VM.  Why do you
think this cost is justified?