[CRIU] [PATCH] criu: cgroup -- Don't use PAGE_SIZE inside parasite args

[Cyrill Gorcunov]

On Thu, Feb 25, 2016 at 08:43:29AM -0700, Tycho Andersen wrote:
> On Thu, Feb 25, 2016 at 10:18:30AM +0300, Cyrill Gorcunov wrote:
> > This cause problem on ppc64
> > 
> >  | gcc -c -O2 -g -Wall -Werror -DCONFIG_PPC64 -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE
> >  | -fno-strict-aliasing -iquote /home/cyrill/criu/criu/include -iquote /home/cyrill/criu/images
> >  | -iquote /home/cyrill/criu/criu/pie -iquote /home/cyrill/criu/criu/arch/ppc64
> >  | -iquote /home/cyrill/criu/criu/arch/ppc64/include -iquote /home/cyrill/criu/ -I/usr/include/libnl3
> >  | -iquote ppc64 -DCONFIG_PPC64 -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE parasite-syscall.c -o parasite-syscall.o
> >  | parasite-syscall.c: In function ‘parasite_dump_cgroup’:
> >  | parasite-syscall.c:1283:2: error: size of unnamed array is negative
> >  |   ca = parasite_args(ctl, struct parasite_dump_cgroup_args);
> >  |   ^
> >  | /home/cyrill/criu/scripts/nmk/scripts/rules.mk:53: recipe for target 'parasite-syscall.o' failed
> > 
> > Just use size parasite known to support.
> > 
> > Signed-off-by: Cyrill Gorcunov <gorcunov at openvz.org>
> 
> Acked-by: Tycho Andersen <tycho.andersen at canonical.com>
> 
> Just for my own education, I thought we always transferred PAGE_SIZE
> number of bytes across for the parasite blob args. If PAGE_SIZE <
> PARASITE_ARG_SIZE_MIN, how does that work?

In turn in the scenario above the page size is greater than 4K, which
lead the bug-on to trigger. Strictly speaking we calculate the area
nedede for parasite arguments during its preparation and it's assumed
that it won't be that big, so the bug-on is rather a note for us that
we potentially doing something wrong if requireing more that 4K for
arguments. We indeed requires memory with

parasite_mmap_exchange
 ...
 ctl->map_length = round_up(size, page_size());

but this is cummulative size of blob itself together with arguments.
That said the only true size used for arguments are PARASITE_ARG_SIZE_MIN,
which is exactly 4k and don't depend on page size.

Something like that.