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Date:   Tue, 9 Aug 2022 18:30:12 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Carlos Llamas <cmllamas@google.com>
Cc:     Liam Howlett <liam.howlett@oracle.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
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        Ondrej Mosnacek <omosnace@redhat.com>,
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        Minchan Kim <minchan@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Hridya Valsaraju <hridya@google.com>,
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        Matthew Wilcox <willy@infradead.org>,
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Subject: Re: [PATCH] binder_alloc: Add missing mmap_lock calls when using
 the VMA
Message-Id: <20220809183012.ac2d52eb91eba55a12d3214a@linux-foundation.org>
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        <20220809190211.ew4taam2lcomramw@revolver>
        <YvLLQ/qasEJffdnn@google.com>
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On Tue, 9 Aug 2022 21:01:55 +0000 Carlos Llamas <cmllamas@google.com> wrote:

> On Tue, Aug 09, 2022 at 07:02:17PM +0000, Liam Howlett wrote:
> > > 
> > > do we need to hold on to the lock while we loop through the pages here?
> > 
> > I think we do?  Holding this lock will ensure the pages don't go away, I
> > believe (looking at mm/rmap.c comments on locking at the top)?
> > 
> > In any case, this function is called from print_binder_proc_stats()
> > which looks to be a debugfs/debugging call so I thought safer would be
> > better than faster and with a potential race.
> 
> The pages are protected by alloc->mutex, so you could immediately
> release the mmap lock after binder_alloc_get_vma() call. I agree this
> is a debugging call so it would be nice to reduce contention.

I'll queue this patch for testing, shall update it if there's a v2.