On Fri, Oct 02, 2020 at 10:45:02AM +0100, Mel Gorman wrote: > On Fri, Oct 02, 2020 at 11:07:29AM +0200, Peter Zijlstra wrote: > > On Fri, Oct 02, 2020 at 09:50:14AM +0100, Mel Gorman wrote: > > > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote: > > > > > > > +#define ___GFP_NO_LOCKS 0x800000u > > > > > > > > Even if a new gfp flag gains a sufficient traction and support I am > > > > _strongly_ opposed against consuming another flag for that. Bit space is > > > > limited. > > > > > > That is definitely true. I'm not happy with the GFP flag at all, the > > > comment is at best a damage limiting move. It still would be better for > > > a memory pool to be reserved and sized for critical allocations. > > > > This is one of the reasons I did a separate allocation function. No GFP > > flag to leak into general usage. > > > > Even a specific function with a hint that "this is for RCU only" will > not prevent abuse. Not exporting it for modules helps, but yes. > > > > Besides that we certainly do not want to allow craziness like > > > > __GFP_NO_LOCK | __GFP_RECLAIM (and similar), do we? > > > > > > That would deserve to be taken to a dumpster and set on fire. The flag > > > combination could be checked in the allocator but the allocator path fast > > > paths are bad enough already. > > > > Isn't that what we have CONFIG_DEBUG_VM for? > > It's enabled by default by enough distros that adding too many checks > is potentially painful. Granted it would be missed by most benchmarking > which tend to control allocations from userspace but a lot of performance > problems I see are the "death by a thousand cuts" variety. Oh quite agreed, aka death by accounting. But if people are enabling DEBUG options in production kernels, there's something wrong, no? Should we now go add CONFIG_REALLY_DEBUG_STAY_AWAY_ALREADY options?
