Re: [PATCH] [BUGFIX] update mm->owner even if no next owner.

[Andrea Arcangeli <aarcange@redhat.com>]

On Sat, Jun 11, 2011 at 09:04:14AM -0700, Hugh Dickins wrote:
> I had another go at reproducing it, 2 hours that time, then a try with
> 692e0b35427a reverted: it ran overnight for 9 hours when I stopped it.
> 
> Andrea, please would you ask Linus to revert that commit before -rc3?
> Or is there something else you'd like us to try instead?  I admit that
> I've not actually taken the time to think through exactly how it goes
> wrong, but it does look dangerous.

Here I was asked if the mem_cgroup_newpage_charge need the mmap_sem at
all. And if not why not to release the mmap_sem early.

https://lkml.org/lkml/2011/3/14/276

So I didn't see why mmap_sem was needed, I also asked confirmation and
who answered agreed it was safe without mmap_sem even if it's the only
place doing that. Maybe that assumption was wrong and we need
mmap_sem after all if this commit is causing problems.

Or did you find something wrong in the actual patch?

Do I understand right that the bug just that we must run
alloc_hugepage_vma+mem_cgroup_newpage_charge within the same critical
section protected by the mmap_sem read mode? Do we know why?

> The way I reproduce it is with my tmpfs kbuilds swapping load,
> in this case restricting mem by memcg, and (perhaps the important
> detail, not certain) doing concurrent swapoff/swapon repeatedly -
> swapoff takes another mm_users reference to the mm it's working on,
> which can cause surprises.

Ok.