On Thu, 15 Jun 2023, Jason Gunthorpe wrote: > On Wed, Jun 14, 2023 at 02:59:33PM -0700, Hugh Dickins wrote: > > > I guess the best thing would be to modify kernel/fork.c to allow the > > architecture to override free_mm(), and arch/s390 call_rcu to free mm. > > But as a quick and dirty s390-end workaround, how about: > > RCU callbacks are not ordered so that doesn't seem like it helps.. Thanks, that's an interesting and important point, which I need to knock into my head better. But can you show me where that's handled in the existing mm/mmu_gather.c include/asm-generic/tlb.h framework? I don't see any rcu_barrier()s there, yet don't the pmd_huge_pte pointers point into pud page tables freed shortly afterwards also by RCU? > > synchronize_rcu would do the job since it is ordered, but I think the > performance cost is too great to just call it from mmdrop Yes, on x86 it proved to be a non-starter; maybe s390 doesn't have the same limitation, but it was clear I was naive to hope that a slowdown on the exit mm path might not be noticeable. > > rcu_barrier() followed by call_rcu on the mm struct might work, but I > don't know the cost SLAB_TYPESAFE_BY_RCU handling has the rcu_barrier() built in, when the slab is destroyed. > > A per-cpu refcount scheme might also do the job reasonably > > Making the page frag pool global (per-cpu global I guess) would also > remove the need to reach back to the freeable mm_struct and reduce the > need for struct page memory. This views it as a special kind of > kmemcache. I haven't thought in that direction at all. Hmm. Or did I think of it once, but discarded for accounting reasons - IIRC (haven't rechecked) page table pages are charged to memcg, and counted for meminfo and other(?) purposes: if the fragments are all lumped into a global pool, we lose that. I think I decided: maybe a good idea, but not a change I should make to get me out of this particular hole. > > Another approach is to not use a rcu_head in the ptdesc at all. > > With a global kmemcache-like-thing we could probably also organize > something where you don't use a rcu_head in the ptdesc, but instead > just a naked 'next' pointer. This would give enough space to have two > next pointers and the next pointers can be re-used for the normal free > list as well. > > In this flow you'd thread the free'd frags onto a waterfall of global > per-cpu lists: > - RCU free the next cycle > - RCU free this cycle > - Actually free > > Where a single rcu_head and single call_rcu frees the entire 2nd list > to the 3rd list and then schedules the 1st list to be RCU'd next. This > eliminates the need to store a function pointer in the ptdesc at > all. > > It requires some global per-cpu lock on the free/alloc paths however, > but this is basically what every other arch does as it frees the page > back to the page allocator. > > I suspect that two next pointers would also eliminate pt_frag_refcount > entirely as we can encode that information in the low bits of the next > pointers. This scheme is clearer in your head than it is in mine. It may be the best solution, but I don't see it clearly enough to judge. I'll carry on with my way, then you can replace it later on. > > > (Funnily enough, there's no problem when the stored mm gets re-used for > > a different mm, once past its spin_lock_init(&mm->context.lock); > > because > > We do that have really weird "type safe by rcu" thing in the > allocators, but I don't quite know how it works. I'm quite familiar with it, since I invented it (SLAB_DESTROY_BY_RCU in 2.6.9 to solve the locking for anon_vma): so it does tend to be my tool of choice when appropriate. It's easy: but you cannot reinitialize the structure on each kmem_cache_alloc(), in particular the spinlocks of the new allocation may have to serve a tail of use from a previous allocation at the same address. > > > Powerpc is like that. I have no idea how much gets wasted that way. > > I was keen not to degrade what s390 does: which is definitely superior, > > but possibly not worth the effort. > > Yeah, it would be good to understand if this is really sufficiently > beneficial.. > > > I'll look into it, once I understand c2c224932fd0. But may have to write > > to Vishal first, or get the v2 of my series out: if only I could work out > > a safe and easy way of unbreaking s390... My latest notion is, just for getting v2 series out, a global spinlock: to be replaced before reaching an actual release. > > Can arches opt in to RCU freeing page table support and still keep > your series sane? Yes, or perhaps we mean different things: I thought most architectures are already freeing page tables by RCU. s390 included. "git grep MMU_GATHER_RCU_TABLE_FREE" shows plenty of selects. > > Honestly, I feel like trying to RCU enable page tables should be its > own series. It is a sufficiently tricky subject on its own right. Puzzled, Hugh
