linux-kernel - Re: [PATCH 07/15] mm: Add ___GFP

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Date:	Mon, 2 Mar 2015 09:58:24 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/15] mm: Add ___GFP_NOTRACE

On Mon, Mar 2, 2015 at 8:46 AM, Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:
> 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@...ux.intel.com> 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, I'm still reading through the patch set.
Quick comment for the above.
Currently there are two map types: array and hash.
array type is pre-allocating all memory at map creation time.
hash is allocating on demand.
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