On Fri, 19 Oct 2012, Glauber Costa wrote:

quoted

quoted

quoted

What about gfp & __GFP_FS?

Do you intend to prevent or allow OOM under that flag? I personally
think that anything that accepts to be OOM-killed should have GFP_WAIT
set, so that ought to be enough.

The oom killer in the page allocator cannot trigger without __GFP_FS 
because direct reclaim has little chance of being very successful and 
thus we end up needlessly killing processes, and that tends to happen 
quite a bit if we dont check for it.  Seems like this would also happen 
with memcg if mem_cgroup_reclaim() has a large probability of failing?

I can indeed see tests for GFP_FS in some key locations in mm/ before
calling the OOM Killer.

Should I test for GFP_IO as well?

It's not really necessary, if __GFP_IO isn't set then it wouldn't make 
sense for __GFP_FS to be set.

If the idea is preventing OOM to
trigger for allocations that can write their pages back, how would you
feel about the following test:

may_oom = (gfp & GFP_KERNEL) && !(gfp & __GFP_NORETRY) ?

I would simply copy the logic from the page allocator and only trigger oom 
for __GFP_FS and !__GFP_NORETRY.

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