On Tue 15-05-18 08:17:27, Pavel Tatashin wrote: > Hi Michal, > > Thank you for your reply, my comments below: > > > You are now disabling a potentially useful feature to SPARSEMEM users > > without having any evidence that they do suffer from the issue which is > > kinda sad. Especially when the only known offender is a UP pcp allocator > > implementation. > > True, but what is the use case for having SPARSEMEM without virtual mapping > and deferred struct page init together. Is it a common case to have > multiple gigabyte of memory and currently NUMA config to benefit from > deferred page init and yet not having a memory for virtual mapping of > struct pages? Or am I missing some common case here? Well, I strongly suspect that this is more a momentum, then a real reason to stick with SPARSEMEM_MANUAL. I would really love to reduce the number of memory models we have. Getting rid of SPARSEMEM would be a good start as VMEMMAP should be much better. > > I will not insist of course but it seems like your fix doesn't really > > prevent virt_to_page or other direct page access either. > > I am not sure what do you mean, I do not prevent virt_to_page, but that is > OK for SPARSEMEM_VMEMMAP case, because we do not need to access "struct > page" for this operation, as translation is in page table. Yes, we do not > prohibit other struct page accesses before mm_init(), but we now have a > feature that checks for uninitialized struct page access, and if those will > happen, we will learn about them. This will always be a maze as the early boot tends to be. Sad but true. That is why I am not really convinced we should use a large hammer and disallow deferred page initialization just because UP implementation of pcp does something too early. We should instead rule that one odd case. Your patch simply doesn't rule a large class of potential issues. It just rules out a potentially useful feature for an odd case. See my point? -- Michal Hocko SUSE Labs
