On Mon, Aug 20, 2012 at 1:00 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > On Fri, 17 Aug 2012, Rik van Riel wrote: >> Of course, that leaves the big question: do we want the >> overhead of having the atomic addition and decrement for >> every anonymous memory page, or is it easier to fix this >> issue in userspace? > > I've not given any thought to alternatives, and I've not done any > performance analysis; but my instinct says that we really do not > want another atomic increment and decrement (and another cache > line redirtied) for every single page mapped. I am concerned about this as well. > May I dare to think: what if we just backed out all the anon_vma_chain > complexity, and returned to the simple anon_vma list we had in 2.6.33? > > Just how realistic was the workload which led you to anon_vma_chains? > And isn't it correct to say that the performance evaluation was made > while believing that each anon_vma->lock was useful, before the sad > realization that anon_vma->root->lock (or ->mutex) had to be used? Thanks for suggesting this - I certainly wish we could go that way. I suspect there will be a strong case against this, but I'd certainly like to hear it (and see if it can be addressed another way). Here we just don't have processes that fork a lot of children that don't immediately exec, so anon_vmas don't bring any value for us. > I've Cc'ed Michel, because I think he has plans (or at least hopes) for > the anon_vmas, in his relentless pursuit of world domination by rbtree. Unfortunately I don't have great ideas there. It would be easy to add a flag to track if an anon_vma has ever been referenced by a struct page, and not clone the anon_vma if the flag isn't set. But, this wouldn't help at all with the DOS potential here. If there are pages referencing the anon_vma, we could reassign these to the parent anon_vma, but finding all such pages would be expensive too. Instead of adding an atomic count for page references, we could limit the anon_vma stacking depth. In fork, we would only clone anon_vmas that have a low enough generation count. I think that's not great (adds a special case for the deep-fork-without-exec behavior), but still better than the atomic page reference counter. I would still prefer if we could just remove the anon_vma_chain stuff, though. -- Michel "Walken" Lespinasse A program is never fully debugged until the last user dies. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>