Re: [RFC PATCH] mm, oom: move GFP_NOFS check to out_of_memory

[Michal Hocko]

On Tue 29-03-16 22:45:40, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > 
> > __alloc_pages_may_oom is the central place to decide when the
> > out_of_memory should be invoked. This is a good approach for most checks
> > there because they are page allocator specific and the allocation fails
> > right after.
> > 
> > The notable exception is GFP_NOFS context which is faking
> > did_some_progress and keep the page allocator looping even though there
> > couldn't have been any progress from the OOM killer. This patch doesn't
> > change this behavior because we are not ready to allow those allocation
> > requests to fail yet. Instead __GFP_FS check is moved down to
> > out_of_memory and prevent from OOM victim selection there. There are
> > two reasons for that
> > 	- OOM notifiers might release some memory even from this context
> > 	  as none of the registered notifier seems to be FS related
> > 	- this might help a dying thread to get an access to memory
> >           reserves and move on which will make the behavior more
> >           consistent with the case when the task gets killed from a
> >           different context.
> 
> Allowing !__GFP_FS allocations to get TIF_MEMDIE by calling the shortcuts in
> out_of_memory() would be fine. But I don't like the direction you want to go.
> 
> I don't like failing !__GFP_FS allocations without selecting OOM victim
> ( http://lkml.kernel.org/r/201603252054.ADH30264.OJQFFLMOHFSOVt@xxxxxxxxxxxxxxxxxxx ).

I didn't get to read and digest that email yet but from a quick glance
it doesn't seem to be directly related to this patch. Even if we decide
that __GFP_FS vs. OOM killer logic is flawed for some reason then would
build on top as granting the access to memory reserves is not against
it.

> Also, I suggested removing all shortcuts by setting TIF_MEMDIE from oom_kill_process()
> ( http://lkml.kernel.org/r/1458529634-5951-1-git-send-email-penguin-kernel@xxxxxxxxxxxxxxxxxxx ).

I personally do not like this much. I believe we have already tried to
explain why we have (some of) those shortcuts. They might be too
optimistic and there is a room for improvements for sure but I am not
convinced we can get rid of them that easily.
-- 
Michal Hocko
SUSE Labs