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

From: Andres Freund
Date: Thu Jan 26 2017 - 18:10:39 EST

Hi Stephane,

On 2017-01-26 14:51:02 -0800, Stephane Eranian wrote:
> Ok, I think I see the problem now (sorry was slow....):

No worries ;)

> the timeline is as follows as seen from the user in your case:
> T0: mmap(0x1000) for func1()
> T1 mmap(0x2000) for func1();
> T3: jit emits info func1() [0x1000-0x1fff]
> T4: mmap(0x3000) for func2()
> T5: mmap(0x4000) for funcs2()
> T6: jit emits info for func2() [0x2000-0x3fff]
> But the problem is that each mmap covers existing mmaps and thus
> supersedes the others as per the time stamp.

Yes, I think that's whats happening. Not that I actually know what I'm
talking about here :)

> The problem is not specific to jit, it just reveals itself in your case.
> The logic in perf is that a more recent mmap supersedes an older one,
> so you have:
> T3: 0x1000-0x2000 owned by func1
> T4: 0x1000-0x3000 owned by anon
> T5: 0x1000-0x4000 owned by anon
> T6: 0x1000-0x4000 owned partially by func2()
> And thus perf cannot symbolize func1() anymore because it has nothing
> mapped in 0x1000-0x1fff but anon.
> Did I get the problem right this time?


> This is tricky to solve here because the tool does not know about the
> merging of the VMAs and assume you are overlapping mmaps and not
> merging them.

Yea, it looked tricky. I'd looked around and the only solutions I'd
found was filtering out the anon mappings (obviously not a real
solution) or preventing the merging (not a real solution either).

> Again the problem is not specific to jit, merging of VMA can happen
> anytime with any app.

Sorry if I hinted in the wrong direction - I didn't see any other bad
consequences. I guess in most other cases with merged VMAs its
relatively harmless, since it'll presumably mostly be memory allocations
and such, where this wont matter.


Andres Freund