On Tue, Mar 14, 2017 at 9:12 AM, Till Smejkal <till.smejkal at googlemail.com> wrote: > On Mon, 13 Mar 2017, Andy Lutomirski wrote: >> On Mon, Mar 13, 2017 at 7:07 PM, Till Smejkal >> <till.smejkal at googlemail.com> wrote: >> > On Mon, 13 Mar 2017, Andy Lutomirski wrote: >> >> This sounds rather complicated. Getting TLB flushing right seems >> >> tricky. Why not just map the same thing into multiple mms? >> > >> > This is exactly what happens at the end. The memory region that is described by the >> > VAS segment will be mapped in the ASes that use the segment. >> >> So why is this kernel feature better than just doing MAP_SHARED >> manually in userspace? > > One advantage of VAS segments is that they can be globally queried by user programs > which means that VAS segments can be shared by applications that not necessarily have > to be related. If I am not mistaken, MAP_SHARED of pure in memory data will only work > if the tasks that share the memory region are related (aka. have a common parent that > initialized the shared mapping). Otherwise, the shared mapping have to be backed by a > file. What's wrong with memfd_create()? > VAS segments on the other side allow sharing of pure in memory data by > arbitrary related tasks without the need of a file. This becomes especially > interesting if one combines VAS segments with non-volatile memory since one can keep > data structures in the NVM and still be able to share them between multiple tasks. What's wrong with regular mmap? > >> >> Ick. Please don't do this. Can we please keep an mm as just an mm >> >> and not make it look magically different depending on which process >> >> maps it? If you need a trampoline (which you do, of course), just >> >> write a trampoline in regular user code and map it manually. >> > >> > Did I understand you correctly that you are proposing that the switching thread >> > should make sure by itself that its code, stack, ? memory regions are properly setup >> > in the new AS before/after switching into it? I think, this would make using first >> > class virtual address spaces much more difficult for user applications to the extend >> > that I am not even sure if they can be used at all. At the moment, switching into a >> > VAS is a very simple operation for an application because the kernel will just simply >> > do the right thing. >> >> Yes. I think that having the same mm_struct look different from >> different tasks is problematic. Getting it right in the arch code is >> going to be nasty. The heuristics of what to share are also tough -- >> why would text + data + stack or whatever you're doing be adequate? >> What if you're in a thread? What if two tasks have their stacks in >> the same place? > > The different ASes that a task now can have when it uses first class virtual address > spaces are not realized in the kernel by using only one mm_struct per task that just > looks differently but by using multiple mm_structs - one for each AS that the task > can execute in. When a task attaches a first class virtual address space to itself to > be able to use another AS, the kernel adds a temporary mm_struct to this task that > contains the mappings of the first class virtual address space and the one shared > with the task's original AS. If a thread now wants to switch into this attached first > class virtual address space the kernel only changes the 'mm' and 'active_mm' pointers > in the task_struct of the thread to the temporary mm_struct and performs the > corresponding mm_switch operation. The original mm_struct of the thread will not be > changed. > > Accordingly, I do not magically make mm_structs look differently depending on the > task that uses it, but create temporary mm_structs that only contain mappings to the > same memory regions. This sounds complicated and fragile. What happens if a heuristically shared region coincides with a region in the "first class address space" being selected? I think the right solution is "you're a user program playing virtual address games -- make sure you do it right". --Andy
