linux-kernel - Re: perf/jit doesn't cope well with mprotect() to jit containing pages

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Date:   Mon, 12 Dec 2016 09:49:03 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andres Freund <andres@...razel.de>
Cc:     linux-kernel@...r.kernel.org,
        Stephane Eranian <eranian@...gle.com>, acme@...nel.org,
        jolsa@...hat.com, mingo@...e.hu, anton@...abs.org,
        namhyung@...nel.org, Stephane Eranian <eranian@...il.com>
Subject: Re: perf/jit doesn't cope well with mprotect() to jit containing
 pages

On Fri, Dec 09, 2016 at 09:02:18PM -0800, Andres Freund wrote:
> 
> I presume the increasing MMAP2 size is triggered by the consecutive
> pages being represented as a single page-range in the kernel?

Yes, we print struct vm_area_struct based information, if vmas get
merged, that shows.

> If I, to work around such consecutive pages, force another page to be
> mmap()ed inbetween, and avoid using MAP_ANONYMOUS, the problem also goes
> away.

This would indeed inhibit vma merging.

> Am I doing something wrong, or is there a bug here?

Expected behaviour afaict

> BTW, it's also a bit weird that those MMAP2 records triggered by
> mprotect/mmap, have prot set to 0...

Yes, mprotect() does: vma->vm_flags = newflags; before calling
perf_event_mmap(vma); which then looks at VM_{READ,WRITE,EXEC} bits in
that word to generate the prot value.

So that being 0 is a bit weird.