Re: [PATCH 0/3] mm/mmap.c: don't unmap the overlapping VMA(s)

[Piotr Kwapulinski]

On Thu, Apr 07, 2016 at 06:31:09PM +0200, Michal Hocko wrote:
> On Thu 07-04-16 18:11:29, Piotr Kwapulinski wrote:
> > 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.
> 
> Yes and this is the standard behavior for ages. So _why_ somebody wants
> non-default behavior. When I've asked for the use case I meant a real
> life code (not just an example snippet) which cannot cope with the
> standard semantic. In other words why this cannot be handled in the
> userspace and we have to add a new API which we have to maintain for
> ever?

Ok, I got it. Thanks for feedback.