Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED file segments

Hugh Dickins
2009-02-02T12:56:10Z

On Mon, 2 Feb 2009, KOSAKI Motohiro wrote:
> >
> > - if (flags & MAP_NORESERVE)
> > + /*
> > + * Set 'VM_NORESERVE' if we should not account for the
> > + * memory use of this mapping. We only honor MAP_NORESERVE
> > + * if we're allowed to overcommit memory.
> > + */
> > + if ((flags & MAP_NORESERVE) && sysctl_overcommit_memory != OVERCOMMIT_NEVER)
>
> I afraid this line a bit.
> if following scenario happend, we can lost VM_NORESERVE?
>
> 1. admin set overcommit_memory to "never"
> 2. mmap
> 3. admin set overcommit_memory to "guess"

I still haven't reviewed it fully myself (and note that what
Linus put in his tree is not identical to this posted patch),
but I do believe this is okay.

When admin changes overcommit_memory, we don't make a pass across
every vma of every mm in the system, to adjust all the accounting
of VM_NORESERVE areas; so I think it's quite reasonable to take
VM_NORESERVE as reflecting the policy in force when that vma was
created. And nothing is displaying the VM_NORESERVE flag.

Ah, you're actually thinking of
4. mprotect
with the original flags (!VM_WRITE) such that no VM_ACCOUNT was done,
and now VM_WRITE is added and the accounting is done despite it having
been mapped MAP_NORESERVE originally. Whereas before Linus's change,
VM_NORESERVE would have still exempted it.

Well... I don't think I care!

But I wonder what the hugetlb situation is: that
if (!accountable)
vm_flags |= VM_NORESERVE;
looks suspicious to me, they look as if they're exempting all
the hugetlb pages from its accounting, whereas !accountable was
only supposed to exempt them from mmap_region()'s own accounting.

Perhaps. I'm still looking at other things,
not given this the time it needs yet.

Hugh