Re: [Linux-ia64] Fix for for memory leak in IA32 mmap

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

>>>>> On Tue, 5 Mar 2002 08:13:01 -0700, Don Dugger <n0ano@n0ano.com> said:

  Don> David- Here is a patch against `linux-2.4.17-ia64-011226.diff'
  Don> that fixes a memory leak with the IA32 `mmap'/`munmap' calls.
  Don> The problem occurs when a non-fixed `mmap' allocates a range
  Don> that ends in the middle of a page.

Nasty.  You're talking about a _virtual_ address space leak, right?  Do
you know the exact sequence of events that causes application failure?

  Don> To handle problems with
  Don> fixed requests the `munmap' call rounds down the the area
  Don> freed, causing the memory leak.  The only solution I can think
  Don> of to deal with this is to create a list of the allocated
  Don> starting addresses for all non-fixed `mmap' requests.  `munmap'
  Don> then checks this list and, if it finds a match, rounds the
  Don> request size up rather than down.

It seems to me what we really want to do is keep track of partially
mapped pages.  I think we'd need a bitmask showing which ia32 pages
have been mapped in an ia64 page.  Say, a 16KB page whose first 4KB
have been mapped would be represented as:

	       ia32 page:
	0	1	2	3

	1	0	0	0

now, if someone maps the 3rd 4KB page, you'd get:

	       ia32 page:
	0	1	2	3

	1	0	1	0

and so on.  The underlying ia64 page would then have to be freed
whenever the bitmask becomes empty.

	--david