Re: [PATCH RFC] rcu/tree: Use GFP_MEMALLOC for alloc memory to free memory pattern

[Joel Fernandes]

On Wed, Apr 1, 2020 at 9:14 AM Mel Gorman <mgorman@xxxxxxx> wrote:
>
> On Wed, Apr 01, 2020 at 09:23:59AM +0200, Michal Hocko wrote:
> > > Can you suggest what prevents other users of GFP_MEMALLOC from doing that
> > > also?
> >
> > There is no explicit mechanism which is indeed unfortunate. The only
> > user real user of the flag is Swap over NFS AFAIK. I have never dared to
> > look into details on how the complete reserves depletion is prevented.
> > Mel would be much better fit here.
> >
>
> It's "prevented" by the fact that every other memory allocation request
> that is not involved with reclaiming memory gets stalled in the allocator
> with only the swap subsystem making any progress until the machine
> recovers. Potentially only kswapd is still running until the system
> recovers if stressed hard enough.
>
> The naming is terrible but is mased on kswapd's use of the PF_MEMALLOC
> flag. For swap-over-nfs, GFP_MEMALLOC saying "this allocation request is
> potentially needed for kswapd to make forward progress and not freeze".
>
> I would not be comfortable with kfree_rcu() doing the same thing because
> there can be many callers in parallel and it's freeing slab objects.
> Swap over NFS should free at least one page, freeing a slab object is
> not guaranteed to free anything.

Got it Mel. Just to clarify to the onlooker. It seemed to fit the
pattern that's why I proposed it as RFC, I was never sure it was the
right approach -- I just proposed it for discussion-sake because I
thought it was worth talking about at least. It was not even merged in
my tree, was just RFC.

Thanks Mel for clarifying the usage of the flag.

 - Joel