shmem: update memory reservation on truncate

shmem: update memory reservation on truncate

A shared anonymous mapping created without MAP_NORESERVE holds memory
reservation for whole range of shmem segment.  Usually there is no way to
change its size, but /proc/<pid>/map_files/...  (available if
CONFIG_CHECKPOINT_RESTORE=y) allows that.

This patch adjusts the memory reservation in shmem_setattr().

exploit:

#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
	unsigned long addr;
	char path[100];

	/* charge 4KiB */
	addr = (unsigned long)mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
	sprintf(path, "/proc/self/map_files/%lx-%lx", addr, addr + 4096);
	truncate(path, 1 << 30);
	/* uncharge 1GiB */
}

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>