Re: [PATCH 10 of 16] stop useless vm trashing while we wait the TIF_MEMDIE task to exit

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Jun 08, 2007 at 08:01:58PM -0700, Christoph Lameter wrote:
> On Sat, 9 Jun 2007, Andrea Arcangeli wrote:
> 
> > I'm sorry to inform you that the oom killing in current mainline has
> > always been a global event not a per-node one, regardless of the fixes
> > I just posted.
> 
> Wrong. The oom killling is a local event if we are in a constrained 
> allocation. The allocating task is killed not a random task. That call to 
> kill the allocating task should not set any global flags.

I just showed the global flag that is being checked. TIF_MEMDIE
affects the whole system, not just your node-constrained allocating
task. If your local constrained task fails to exit because it's
running in the nfs path that loops forever even if NULL is returned
from alloc_pages, it will deadlock the whole system if later a regular
oom happens (alloc_pages isn't guaranteed to be called by a page fault
where we know do_exit will guaranteed to be called if a sigkill is
pending). This is just an example.

Amittedly my fixes made things worse for your "local" oom killing, but
your code was only apparently "local" because TIF_MEMDIE is a _global_
flag in the mainline kernel. So again, I'm very willing to improve the
local oom killing, so that it will really become a local event for the
first time ever. Infact with my fixes applied the whole system will
stop waiting for the TIF_MEMDIE flag to go away, so it'll be much
easier to really make the global oom killing independent from the
local one. I didn't look into the details of the local oom killing yet
(exactly because it wasn't so local in the first place) but it may be
enough to set VM_is_OOM only for tasks that are not being locally
killed and then those new changes will automatically prevent
TIF_MEMDIE being set on a local-oom to affect the global-oom event.

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