On Fri 02-10-20 09:50:14, Mel Gorman wrote: > On Fri, Oct 02, 2020 at 09:11:23AM +0200, Michal Hocko wrote: > > On Thu 01-10-20 21:26:26, Uladzislau Rezki wrote: > > > > > > > > No, I meant going back to idea of new gfp flag, but adjust the implementation in > > > > the allocator (different from what you posted in previous version) so that it > > > > only looks at the flag after it tries to allocate from pcplist and finds out > > > > it's empty. So, no inventing of new page allocator entry points or checks such > > > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't > > > > affect existing fast paths. > > > > > > > OK. Now i see. Please have a look below at the patch, so we fully understand > > > each other. If that is something that is close to your view or not: > > > > > > <snip> > > > t a/include/linux/gfp.h b/include/linux/gfp.h > > > index c603237e006c..7e613560a502 100644 > > > --- a/include/linux/gfp.h > > > +++ b/include/linux/gfp.h > > > @@ -39,8 +39,9 @@ struct vm_area_struct; > > > #define ___GFP_HARDWALL 0x100000u > > > #define ___GFP_THISNODE 0x200000u > > > #define ___GFP_ACCOUNT 0x400000u > > > +#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. Completely agreed. The only existing usecase is so special cased that a dedicated pool is not only easier to maintain but it should be also much better tuned for the specific workload. Something not really feasible with the allocator. > > 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. If a new allocation/gfp mode is absolutely necessary then I believe that the most reasoanble way forward would be #define GFP_NO_LOCK ((__force gfp_t)0) and explicitly document it as a final flag to use without any further modifiers. Yeah there are some that could be used potentially - e.g. zone specifiers, __GFP_ZERO and likely few others. But support for those can be added when there is an actual and reasonable demand. I would also strongly argue against implementation alowing to fully consume pcp free pages. -- Michal Hocko SUSE Labs
