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 PERF_RECORD_KSYMBOL. 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. Did you read this proposal? https://lkml.kernel.org/r/20190109101808.gg1...@hirez.programming.kicks-ass.net (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.