On Thu, Apr 08, 2021 at 01:37:34AM -0700, Michel Lespinasse wrote: > On Thu, Apr 08, 2021 at 08:13:43AM +0100, Matthew Wilcox wrote: > > On Thu, Apr 08, 2021 at 09:00:26AM +0200, Peter Zijlstra wrote: > > > On Wed, Apr 07, 2021 at 10:27:12PM +0100, Matthew Wilcox wrote: > > > > Doing I/O without any lock held already works; it just uses the file > > > > refcount. It would be better to use a vma refcount, as I already said. > > > > > > The original workload that I developed SPF for (waaaay back when) was > > > prefaulting a single huge vma. Using a vma refcount was a total loss > > > because it resulted in the same cacheline contention that down_read() > > > was having. > > > > > > As such, I'm always incredibly sad to see mention of vma refcounts. > > > They're fundamentally not solving the problem :/ > > > > OK, let me outline my locking scheme because I think it's rather better > > than Michel's. The vma refcount is the slow path. > > > > 1. take the RCU read lock > > 2. walk the pgd/p4d/pud/pmd > > 3. allocate page tables if necessary. *handwave GFP flags*. > > 4. walk the vma tree > > 5. call ->map_pages > > 6. take ptlock > > 7. insert page(s) > > 8. drop ptlock > > if this all worked out, we're done, drop the RCU read lock and return. > > 9. increment vma refcount > > 10. drop RCU read lock > > 11. call ->fault > > 12. decrement vma refcount > > Note that most of your proposed steps seem similar in principle to mine. > Looking at the fast path (steps 1-8): > - step 2 sounds like the speculative part of __handle_mm_fault() > - (step 3 not included in my proposal) > - step 4 is basically the lookup I currently have in the arch fault handler > - step 6 sounds like the speculative part of map_pte_lock() > > I have working implementations for each step, while your proposal > summarizes each as a point item. It's not clear to me what to make of it; > presumably you would be "filling in the blanks" in a different way > than I have but you are not explaining how. Are you suggesting that > the precautions taken in each step to avoid races with mmap writers > would not be necessary in your proposal ? if that is the case, what is > the alternative mechanism would you use to handle such races ? I don't know if you noticed, I've been a little busy with memory folios. I did tell you that on the call, but you don't seem to retain anything I tell you on the call, so maybe I shouldn't bother calling in any more. > Going back to the source of this, you suggested not copying the VMA, > what is your proposed alternative ? Do you suggest that fault handlers > should deal with the vma potentially mutating under them ? Or should > mmap writers consider vmas as immutable and copy them whenever they > want to change them ? or are you implying a locking mechanism that would > prevent mmap writers from executing while the fault is running ? The VMA should be immutable, as I explained to you before.
