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SubjectRe: [PATCH 0/3] mm/mmap.c: don't unmap the overlapping VMA(s)
On Mon, Apr 04, 2016 at 05:26:43PM +0200, Vlastimil Babka wrote:
> On 04/04/2016 09:31 AM, Michal Hocko wrote:
> >On Sat 02-04-16 21:17:31, Piotr Kwapulinski wrote:
> >>Currently the mmap(MAP_FIXED) discards the overlapping part of the
> >>existing VMA(s).
> >>Introduce the new MAP_DONTUNMAP flag which forces the mmap to fail
> >>with ENOMEM whenever the overlapping occurs and MAP_FIXED is set.
> >>No existing mapping(s) is discarded.
> >
> >You forgot to tell us what is the use case for this new flag.
>
> Exactly. Also, returning ENOMEM is strange, EINVAL might be a better match,
> otherwise how would you distinguish a "geunine" ENOMEM from passing a wrong
> address?
>
>

Thanks to all for suggestions. I'll fix them.

The example use case:
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>

void main(void)
{
void* addr = (void*)0x1000000;
size_t size = 0x600000;
void* start = 0;
start = mmap(addr,
size,
PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
-1, 0);

strcpy(start, "PPPP");
printf("%s\n", start); // == PPPP

addr = (void*)0x1000000;
size = 0x9000;
start = mmap(addr,
size,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
-1, 0);

printf("%s\n", start); // != PPPP
}

Another use case, this time with huge pages in action.
The limit configured in proc's nr_hugepages is exceeded.
mmap unmaps the area and fails. No new mapping is created.
The program segfaults.

echo 0 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages

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

void main(void)
{
void* addr = (void*)0x1000000;
size_t size = 0x600000;
void* start = 0;
start = mmap(addr,
size,
PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED,
-1, 0);

strcpy(start, "PPPP");
printf("%s\n", start); // == PPPP

addr = (void*)0x1000000;
size = 0x400000;
start = mmap(addr,
size,
PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED | MAP_HUGETLB,
-1, 0); // mmap fails but unmaps the area

printf("%s\n", addr); // segfault
}

Piotr

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