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 ? 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 ? > Compared to today, where we bump the refcount on the file underlying the > vma, this is _better_ scalability -- different mappings of the same file > will not contend on the file's refcount. > > I suspect your huge VMA was anon, and that wouldn't need a vma refcount > as faulting in new pages doesn't need to do I/O, just drop the RCU > lock, allocate and retry.
