Re: [RFC PATCH] perf, bpf: Retain kernel executable code in memory to aid Intel PT tracing

From: Adrian Hunter
Date: Fri Feb 08 2019 - 03:55:11 EST

On 7/02/19 10:02 PM, Peter Zijlstra wrote:
> On Thu, Feb 07, 2019 at 01:19:01PM +0200, Adrian Hunter wrote:
>> Subject to memory pressure and other limits, retain executable code, such
>> as JIT-compiled bpf, in memory instead of freeing it immediately it is no
>> longer needed for execution.
>> While perf is primarily aimed at statistical analysis, tools like Intel
>> PT can aim to provide a trace of exactly what happened. As such, corner
>> cases that can be overlooked statistically need to be addressed. For
>> example, there is a gap where JIT-compiled bpf can be freed from memory
>> before a tracer has a chance to read it out through the bpf syscall.
>> While that can be ignored statistically, it contributes to a death by
>> 1000 cuts for tracers attempting to assemble exactly what happened. This is
>> a bit gratuitous given that retaining the executable code is relatively
>> simple, and the amount of memory involved relatively small. The retained
>> executable code is then available in memory images such as /proc/kcore.
>> This facility could perhaps be extended also to init sections.
>> Note that this patch is compile tested only and, at present, is missing
>> the ability to retain symbols.
> You don't need the symbols; you already have them through

And you intend to use that for module loading/unloading also?

> Also; afaict this patch guarantees exactly nothing. It registers a
> shrinker which will (given enough memory pressure) happily free your
> text before we get around to copying it out.

No, there is a minimum size (default 0) which is not subject to the shrinker.

> Did you read this proposal?

Please cc me on anything affecting Intel PT decoding.

> (also: s/KCORE_QC/KCORE_QS/ for quiescent state)
> That would create an RCU like interface to /proc/kcore and give you the
> guarantees you need, while also allowing the memory to get freed once
> you've obtained a copy.

So, open /proc/kcore and it pins all executable code in memory?

Do you intend to extend that to module / module init unloads?