[CRIU] error in VDSO remap

[Tycho Andersen]

On Thu, Jun 04, 2015 at 12:33:33PM +0300, Pavel Emelyanov wrote:
> On 06/04/2015 02:16 AM, Tycho Andersen wrote:
> > Hi Pavel,
> > 
> > On Tue, Jun 02, 2015 at 08:21:17AM +0300, Pavel Emelyanov wrote:
> >> On 06/02/2015 06:39 AM, Tycho Andersen wrote:
> >>> Hi all,
> >>>
> >>> When I try and c/r with current trunk, sometimes I get an error in the
> >>> vdso remap code. The error message looks garbled (I am using current
> >>> trunk), so I'm not sure what's really going on. Two examples are:
> >>>
> >>> http://paste.ubuntu.com/11510594/
> >>> http://paste.ubuntu.com/11511094/
> >>>
> >>> This doesn't happen every time, but maybe 30% of the time.
> >>>
> >>> Any ideas?
> >>
> >> In the kernel code I can find only one reason for the EFAULT from
> >> mremap -- the region we're trying to remap is not strictly included
> >> into some single vma. IOW inside the (addr, addr + size) region
> >> there should be either one whole vma or its part, but no two vmas
> >> and no holes in the start or end of it.
> >>
> >> Can you stop (sleep 1000) the restoring task right after this message
> >> and look inside its /proc/self/maps to compare what it remaps vs
> >> what it really has?
> > 
> > Here's the maps:
> > 
> > http://paste.ubuntu.com/11552302/
> > 
> > and here's my log:
> > 
> > http://paste.ubuntu.com/11552304/
> 
> Hm... It looks like these logs are succeeding:
> 
> pie: vdso: Remap dumpee 0x26000 -> 0x7ffd61fdd000
> pie: vdso: Remap dumpee 0x28000 -> 0x7ffd61fdf000
> pie: Restoring scheduler params 0.0.0
> 
> > It looks like it's trying to map something onto the end of the last
> > region? 
> 
> Well, yes. I have one idea of what can happen here. It looks like the 
> VDSO CRIU sees in the image has managed to merge with some adjacent VMA
> (because of the flags match) so we can't remap it into proper place because
> of the sys_mremap limitations.
> 
> Can you try to catch the failing case to see what CRIU remaps and what
> mappings it has at that time?

Ah, sorry, the error is actually a bit above (search for "Error"); the
sleep made it so that things didn't die right away :)

> > I'm still having trouble parsing the error message, as it
> > doesn't really match the format string.
> 
> You mean this part:
> 
> pie: vdso: image [vdso] 0x7ffd61fdf000-0x7ffd61fe1000 [vvar] 0x7ffd61f>
> pie: dd000-0x7ffd61fdf000
> 
> ?
> 
> That's pie's logging engine :) Since the buffer is quite limited there we
> split lines and mark the split point with ">" sign :)

Oh, right, I forgot about the >; I was referring to:

pie: Error (arch/x86/vdso-pie.c:259): vdso: Unable to remap 0x28000 ->>
pie:  0x7ffd61fdf000 0xfffffffffffffff2

but now it makes sense.

Tycho

> -- Pavel
>