On Wed 07-10-20 00:25:29, Uladzislau Rezki wrote: > On Mon, Oct 05, 2020 at 05:41:00PM +0200, Michal Hocko wrote: > > On Mon 05-10-20 17:08:01, Uladzislau Rezki wrote: > > > On Fri, Oct 02, 2020 at 11:05:07AM +0200, Michal Hocko wrote: > > > > 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) > > > > > > > Agree. Even though i see that some code should be adjusted for it. There are > > > a few users of the __get_free_page(0); So, need to double check it: > > > > Yes, I believe I have pointed that out in the previous discussion. > > > OK. I spent more time on it. A passed gfp_mask can be adjusted on the entry, > that adjustment depends on the gfp_allowed_mask. It can be changed in run-time. > > For example during early boot it excludes: __GFP_RECLAIM|__GFP_IO|__GFP_FS flags, > what is GFP_KERNEL. So, GFP_KERNEL is adjusted on entry and becomes 0 during early > boot phase. Honestly I am not sure how much is GFP_BOOT_MASK still needed. The remaining user of gfp_allowed_mask mask should be only hibernation and I believe this should be removed in long term. Not as trivial because scope API cannot be used for that as it needs a global flag but this is a gross hack that should be implemented differently. It is waiting on my todo list but never got around to that. > How to distinguish it: > > <snip> > + /* > + * gfp_mask can become zero because gfp_allowed_mask changes in run-time. > + */ > + if (!gfp_mask) > + alloc_flags |= ALLOC_NO_LOCKS; > + > gfp_mask &= gfp_allowed_mask; > alloc_mask = gfp_mask; > if (!prepare_alloc_pages(gfp_mask, order, preferred_nid, nodemask, &ac, &alloc_mask, &alloc_flags)) > <snip> > > > > > > > Apart of that. There is a post_alloc_hook(), that gets called from the prep_new_page(). > > > If "debug page alloc enabled", it maps a page for debug purposes invoking kernel_map_pages(). > > > __kernel_map_pages() is ARCH specific. For example, powerpc variant uses sleep-able locks > > > what can be easily converted to raw variant. > > > > Yes, there are likely more surprises like that. I am not sure about > > kasan, page owner (which depens on the stack unwinder) and others which > > hook into this path. > > > I have checked kasan_alloc_pages(), kernel_poison_pages() both are OK, > at least i did not find any locking there. As for set_page_owner(), it > requires more attention, since it uses arch specific unwind logic. Though, > i spent some time on it and so far have not noticed anything. stack depod depends on a lock IIRC. Anyway, this is just showing how this is going to grow in complexity and make future additions harder. A niche usecase is inducing an additional complexity for future maintenance. -- Michal Hocko SUSE Labs
