Re: [PATCH] mm: fix incorrect behavior when process virtual address space limit is exceeded

[Michal Hocko <mhocko@kernel.org>]

On Mon 16-11-15 21:52:10, Michal Hocko wrote:
> [CCing Cyril]
> 
> On Mon 16-11-15 18:36:19, Piotr Kwapulinski wrote:
> > When a new virtual memory area is added to the process's virtual address
> > space and this vma causes the process's virtual address space limit
> > (RLIMIT_AS) to be exceeded then kernel behaves incorrectly. Incorrect
> > behavior is a result of a kernel bug. The kernel in most cases
> > unnecessarily scans the entire process's virtual address space trying to
> > find the overlapping vma with the virtual memory region being added.
> > The kernel incorrectly compares the MAP_FIXED flag with vm_flags variable
> > in mmap_region function. The vm_flags variable should not be compared
> > with MAP_FIXED flag. The MAP_FIXED flag has got the same numerical value
> > as VM_MAYREAD flag (0x10). As a result the following test
> > from mmap_region:
> > 
> > if (!(vm_flags & MAP_FIXED))
> > is in fact:
> > if (!(vm_flags & VM_MAYREAD))
> 
> It seems this has been broken since it was introduced e8420a8ece80
> ("mm/mmap: check for RLIMIT_AS before unmapping"). The patch has fixed
> the case where unmap happened before the we tested may_expand_vm and
> failed which was sufficient for the LTP test case but it apparently
> never tried to consider the overlapping ranges to eventually allow to
> create the mapping.
> 
> > The VM_MAYREAD flag is almost always set in vm_flags while MAP_FIXED
> > flag is not so common. The result of the above condition is somewhat
> > reverted.
> > This patch fixes this bug. It causes that the kernel tries to find the
> > overlapping vma only when the requested virtual memory region has got
> > the fixed starting virtual address (MAP_FIXED flag set).
> > For tile architecture Calling mmap_region with the MAP_FIXED flag only is
> > sufficient. However the MAP_ANONYMOUS and MAP_PRIVATE flags are passed for
> > the completeness of the solution.
> 
> I found the changelog rather hard to grasp. But the fix is correct. The
> user visible effect is that RLIMIT_AS might hit for a MAP_FIXED
> mapping even though it wouldn't enlarge the used address space.

And I was obviously blind and read the condition incorrectly. Sigh...
There is no real issue in fact. We do call count_vma_pages_range even
for !MAP_FIXED case but that is merely a pointless find_vma call but
nothing really earth shattering. So nothing for the stable tree and also
quite exaggerated to be called a bug.

I am sorry about the confusion.
-- 
Michal Hocko
SUSE Labs