linux-kernel - Re: [PATCH] Add mmap_assert_locked() annotations to find

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Date:   Sun, 1 Aug 2021 12:33:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Luigi Rizzo <lrizzo@...gle.com>
Cc:     linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
        linux-mm@...ck.org
Subject: Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()

On Sat, 31 Jul 2021 10:53:41 -0700 Luigi Rizzo <lrizzo@...gle.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?