On Fri, Dec 06, 2019 at 12:30:30PM -0500, Jerome Glisse wrote: > On Thu, Dec 05, 2019 at 09:13:22PM -0800, Matthew Wilcox wrote: > > On Thu, Dec 05, 2019 at 12:21:50PM -0500, Jerome Glisse wrote: > > > On Tue, Dec 03, 2019 at 02:21:47PM -0800, Matthew Wilcox wrote: > > > > While one thread is calling mmap(MAP_FIXED), two other threads which are > > > > accessing the same address may see different data from each other and > > > > have different page translations in their respective CPU caches until > > > > the thread calling mmap() returns. I believe this is OK, but would > > > > greatly appreciate hearing from people who know better. > > > > > > I do not believe this is OK, i believe this is wrong (not even considering > > > possible hardware issues that can arise from such aliasing). > > > > Well, OK, but why do you believe it is wrong? If thread A is executing > > a load instruction at the same time that thread B is calling mmap(), > > it really is indeterminate what value A loads. It might be from before > > the call to mmap() and it might be from after. And if thread C is also > > executing a load instruction at the same time, then it might already get > > a different result from thread A. And can threads A and C really tell > > which of them executed the load instruction 'first'? I think this is > > all so indeterminate already that the (lack of) guarantees I outlined > > above are acceptable. > > > > But we should all agree on this, so _please_ continue to argue your case > > for why you believe it to be wrong. > > I agree that such application might looks like it is doing something that > is undeterminate but their might be application that catch SEGFAULT and use > it as synchronization. I did something similar for reverse engineering a > long time ago with a library call libsegfault ... > > In any case, i agree that an application that is not catching SEGFAULT, and > which is doing the above (access patterns) is doing something undeterminate. > > Nonetheless i believe it is important that at any point in time for all the > threads in a given process, on all the CPUs, a given virtual address should > always point to the same physical memory (or to nothing) ie we should never > have one CPU that sees a different physical memory from another CPU for the > same virtual address. > > Well i feel like you are also not discussing about the munmap() the above > seemed to be about MAP_FIXED (replacing an existing mapping). For munmap > too i believe we should agree on what should be the expected behavior and > from my POV again we should not allow new mapping to appear until a "running" > munmap is not fully done (ie all CPUs cache and TLB flushed). For the same > reason as above ie all CPUs always see same physical memory (or nothing) for > a given virtual address. I see MAP_FIXED as being the harder case, but sure, let's talk about munmap! I agree that a munmap() + mmap() call should not permit thread B to see the old value after thread A has seen the new value. But, as long as no new mmap can occupy that range, then it's OK if thread A takes a segfault while thread B can still load the old value. At least for a short window. We can replicate that behaviour by ensuring that new lookups see a NULL entry, but new attempts to allocate will not reuse that range until the munmap has finished and all TLB entries are flushed. The maple tree actually supports a "ZERO" entry (just like the XArray does) which has this behaviour -- lookups see NULL, but attempts to allocate do not see it as free. We already use that property to prevent allocating above the end of the process address space. > This is what we have today with the big rwsem and i think we need to keep > that behavior even with concurency. I do not believe this will impact the > performance and it is easy enough to solve so i feel safer doing so given > it does not cost anything. > > So i would rather argue on why we should change the current behavior if we > can fix the concurrency without changing it (hence why discussing solution > might also be relevant here). It seems like you want to force a thread which sees an ongoing munmap to spin or sleep until the munmap is done, rather than immediately take a segfault, and I don't know that's a useful behaviour. > > > Just to make sure i understand, you propose that ->map_pages() becomes > > > a new ->fault() handler that get calls before ->fault() without refcount > > > so that we can update fs/drivers slowly to perform better in the new scheme > > > (ie avoid the overead of refcounting if possible at all) ? > > > > > > The ->fault() callback would then be the "slow" path which will require > > > a refcount on the vma (taken by core mm code before dropping rcu lock). > > > > I would actually propose never updating most drivers. There's just no > > need for them to handle such an unstable and tricky situation as this. > > Let's not make driver writers lives harder. > > > > For the ones which need this kind of scalability (and let's be clear, they > > would already have *better* scalability than today due to the rwsem being > > split into a per-VMA refcount), then yes, implementing ->map_pages would > > be the way to go. Indeed, they would probably benefit from implementing > > it today, since it will reduce the number of page faults. > > Yes they will get better scalability but i see some of those drivers might > want the extra few mini-percent :) In any case, i believe that it might be > better to give a new name ie fix current map_pages() user and rename that > callback to something more explicit (atomic_map_pages() or something similar > i am not good at naming). But otherwise this looks like a good plan to avoid > excessive refcount overhead. OK, great. I don't think the current name is bad, but if someone comes up with a better one, I don't have a problem with renaming it.
