Re: [patch] mm, thp: do not cause memcg oom for thp

[David Rientjes]

On Wed, 21 Mar 2018, Michal Hocko wrote:

> > That doesn't make sense, the allocation path needs to allocate contiguous 
> > memory for the high order, the charging path just needs to charge a number 
> > of pages.  Why would the allocation and charging path be compatible when 
> > one needs to reclaim contiguous memory or compact memory and the the other 
> > just needs to reclaim any memory?
> 
> Because you do not want to see surprises. E.g. seeing unexpected OOMs
> for large allocatations. Just think about it. Do you really want to have
> a different reclaim policy for the allocation and charging for all
> allocating paths?

It depends on the use of __GFP_NORETRY.  If the high-order charge is 
__GFP_NORETRY, it does not oom kill.  It is left to the caller.  Just 
because thp allocations have been special cased in the page allocator to 
be able to remove __GFP_NORETRY without fixing the memcg charge path does 
not mean memcg needs a special heuristic for high-order memory when it 
does not require contiguous memory.  You say you don't want any surprises, 
but now you are changing behavior needlessly for all charges with
order > PAGE_ALLOC_COSTLY_ORDER that do not use __GFP_NORETRY.

> You are right that the allocation path involves compaction and that is
> different from the charging path. But that is an implementation detail
> of the current implementation.
> 

Lol, the fact that the page allocator requires contiguous memory is not an 
implementation detail of the current implementation.

> Your patch only fixes up the current situation. Anytime a new THP
> allocation emerges that code path has to be careful to add
> __GFP_NORETRY to not regress again. That is just too error prone.
> 

We could certainly handle it by adding helpers similar to 
alloc_hugepage_direct_gfpmask() and alloc_hugepage_khugepaged_gfpmask() 
which are employed for the same purpose for the page allocator gfp mask.