Re: MAP_NORESERVE with MAP_SHARED

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On 12 Jul 2002 23:15:02 +0100, Alan Cox <alan@lxorguk.ukuu.org.uk> said:

  Alan> On Fri, 2002-07-12 at 21:39, David Mosberger wrote:
  >> Is there a good reason why the MAP_NORESERVE flag is ignored when
  >> MAP_SHARED is specified?  (Hint: it's the call to vm_enough_memory()
  >> in shmem_file_setup() that's causing MAP_NORESERVE to be ignored.)

  Alan> In no overcommit mode MAP_NORESERVE is never honoured. In conventional
  Alan> overcommit mode I may have broken something between base and -ac. Which
  Alan> bit of the code are you looking at ?

2.4.18, though as far as I looked, the latest 2.5 has the same behavior.
Specifically, in do_mmap_pgoff() we have:

	/* Private writable mapping? Check memory availability.. */
	if ((vm_flags & (VM_SHARED | VM_WRITE)) == VM_WRITE &&
	    !(flags & MAP_NORESERVE)				 &&
	    !vm_enough_memory(len >> PAGE_SHIFT))
		return -ENOMEM;

and in shmem_file_setup() we have:

	if (!vm_enough_memory((size) >> PAGE_CACHE_SHIFT))
		return ERR_PTR(-ENOMEM);

So, if we don't have MAP_SHARED (first case), MAP_NORESERVE is
honored, whereas with MAP_SHARED (second case), MAP_NORESERVE is
ignored.

	--david