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
