Re: [PATCH 07/15] mm: Add ___GFP_NOTRACE

[Tom Zanussi]

On Mon, 2015-03-02 at 11:37 -0500, Steven Rostedt wrote:
> On Mon,  2 Mar 2015 10:01:00 -0600
> Tom Zanussi <tom.zanussi@xxxxxxxxxxxxxxx> wrote:
> 
> > Add a gfp flag that allows kmalloc() et al to be used in tracing
> > functions.
> > 
> > The problem with using kmalloc for tracing is that the tracing
> > subsystem should be able to trace kmalloc itself, which it can't do
> > directly because of paths like kmalloc()->trace_kmalloc()->kmalloc()
> > or kmalloc()->trace_mm_page_alloc()->kmalloc().
> 
> This part I don't like at all. Why can't the memory be preallocated
> when the hist is created (the echo 'hist:...')?
> 

Yeah, I didn't like it either.  My original version did exactly what you
suggest and preallocated an array of entries to 'allocate' from in order
to avoid the problem.

But I wanted to attempt to use the bpf_map directly, which already uses
kmalloc internally.  My fallback in case this wouldn't fly, which it
obviously won't, would be to add an option to have the bpf_map code
preallocate a maximum number of entries or pass in a client-owned array
for the purpose.  I'll do that if I don't hear any better ideas..

Tom

> kmalloc must never be called from any tracepoint callback.
> 
> This change is currently a showstopper.
> 
> -- Steve
> 
> 
> > 
> > With this flag, tracing code could use a special version of kmalloc()
> > that sets __GFP_NOTRACE on every allocation it does, while leaving the
> > normal kmalloc() path untouched.
> > 
> > This would allow any tracepoint in the kmalloc path to be avoided via
> > DEFINE_EVENT_CONDITION() redefinitions of those events, which check
> > for ___GFP_NOTRACE immediately in their execution and break if set,
> > thereby avoiding the recursion.
> > 


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