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

[Tetsuo Handa]

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 ).

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 ).

> 
> Keep a comment in __alloc_pages_may_oom to make sure we do not forget
> how GFP_NOFS is special and that we really want to do something about
> it.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> 
> Hi,
> I am sending this as an RFC now even though I think this makes more
> sense than what we have right now. Maybe there are some side effects
> I do not see, though. A more tricky part is the OOM notifier part
> becasue future notifiers might decide to depend on the FS and we can
> lockup. Is this something to worry about, though? Would such a notifier
> be correct at all? I would call it broken as it would put OOM killer out
> of the way on the contended system which is a plain bug IMHO.
> 
> If this looks like a reasonable approach I would go on think about how
> we can extend this for the oom_reaper and queue the current thread for
> the reaper to free some of the memory.
> 
> Any thoughts