Re: [PATCH] tracing: (backport) Replace kmap with copy_from_user() in trace_marker

[Greg KH]

On Fri, Dec 09, 2016 at 09:05:51AM +0100, Henrik Austad wrote:
> On Fri, Dec 09, 2016 at 08:22:05AM +0100, Greg KH wrote:
> > 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@xxxxxxxxxx>
> > > Cc: Henrik Austad <henrik@xxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > 
> > > Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Used-to-be-signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > > Backported-by: Henrik Austad <haustad@xxxxxxxxx>
> > > Tested-by: Henrik Austad <haustad@xxxxxxxxx>
> > > Signed-off-by: Henrik Austad <haustad@xxxxxxxxx>
> > > ---
> > >  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?
> 
> Ah, perhaps I jumped the gun here. I don't think Linus has picked this one 
> up yet, Steven sent out the patch yesterday.
> 
> Since then, I've backported it to 3.10 and ran the first set of tests 
> over night and it looks good. So ideally this would find its way into 
> 3.10(.104).
> 
> Do you want med to resubmit when Stevens patch is merged upstream?

Yes please, we can't do anything until it is in Linus's tree, please see
Documentation/stable_kernel_rules.txt for how this all works.

thanks,

greg k-h