linux-kernel - Re: [PATCH] tracing: (backport) Replace kmap with copy_from_user() in trace

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Message-ID: <20161209072205.GC29639@kroah.com>
Date:   Fri, 9 Dec 2016 08:22:05 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Henrik Austad <henrik@...tad.us>
Cc:     linux-kernel@...r.kernel.org, Henrik Austad <haustad@...co.com>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>, stable@...r.kernel.org
Subject: Re: [PATCH] tracing: (backport) Replace kmap with copy_from_user()
 in trace_marker

On Fri, Dec 09, 2016 at 07:34:04AM +0100, Henrik Austad wrote:
> Instead of using get_user_pages_fast() and kmap_atomic() when writing
> to the trace_marker file, just allocate enough space on the ring buffer
> directly, and write into it via copy_from_user().
> 
> Writing into the trace_marker file use to allocate a temporary buffer
> to perform the copy_from_user(), as we didn't want to write into the
> ring buffer if the copy failed. But as a trace_marker write is suppose
> to be extremely fast, and allocating memory causes other tracepoints to
> trigger, Peter Zijlstra suggested using get_user_pages_fast() and
> kmap_atomic() to keep the user space pages in memory and reading it
> directly.
> 
> Instead, just allocate the space in the ring buffer and use
> copy_from_user() directly. If it faults, return -EFAULT and write
> "<faulted>" into the ring buffer.
> 
> On architectures without a arch-specific get_user_pages_fast(), this
> will end up in the generic get_user_pages_fast() and this grabs
> mm->mmap_sem. Once you do this, then suddenly writing to the
> trace_marker can cause priority-inversions.
> 
> This is a backport of Steven Rostedts patch [1] and applied to 3.10.x so the
> signed-off-chain by is somewhat uncertain at this stage.
> 
> The patch compiles, boots and does not immediately explode on impact. By
> definition [2] it must therefore be perfect
> 
> 2) https://www.spinics.net/lists/kernel/msg2400769.html
> 2) http://lkml.iu.edu/hypermail/linux/kernel/9804.1/0149.html
> 
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Henrik Austad <henrik@...tad.us>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: stable@...r.kernel.org
> 
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Used-to-be-signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> Backported-by: Henrik Austad <haustad@...co.com>
> Tested-by: Henrik Austad <haustad@...co.com>
> Signed-off-by: Henrik Austad <haustad@...co.com>
> ---
>  kernel/trace/trace.c | 78 +++++++++++++++-------------------------------------
>  1 file changed, 22 insertions(+), 56 deletions(-)

What is the git commit id of this patch in Linus's tree?  And what
stable trees do you feel it should be applied to?

thanks,

greg k-h