On Tue 30-05-17 17:43:26, Andrea Arcangeli wrote: > On Tue, May 30, 2017 at 04:39:41PM +0200, Michal Hocko wrote: > > I sysctl for the mapcount can be increased, right? I also assume that > > those vmas will get merged after the post copy is done. > > Assuming you enlarge the sysctl to the worst possible case, with 64bit > address space you can have billions of VMAs if you're migrating 4T of > RAM and you're unlucky and the address space gets fragmented. The > unswappable kernel memory overhead would be relatively large > (i.e. dozen gigabytes of RAM in vm_area_struct slab), and each > find_vma operation would need to walk ~40 steps across that large vma > rbtree. There's a reason the sysctl exist. Not to tell all those > unnecessary vma mangling operations would be protected by the mmap_sem > for writing. > > Not creating a ton of vmas and enabling vma-less pte mangling with a > single large vma and only using mmap_sem for reading during all the > pte mangling, is one of the primary design motivations for > userfaultfd. Yes, I am aware of fallouts of too many vmas. I was asking merely to learn whether this will really happen under the the specific usecase Mike is after. > > I understand that part but it sounds awfully one purpose thing to me. > > Are we going to add other MADVISE_RESET_$FOO to clear other flags just > > because we can race in this specific use case? > > Those already exists, see for example MADV_NORMAL, clearing > ~VM_RAND_READ & ~VM_SEQ_READ after calling MADV_SEQUENTIAL or > MADV_RANDOM. I would argue that MADV_NORMAL is everything but a clear madvise command. Why doesn't it clear all the sticky MADV* flags? > Or MADV_DOFORK after MADV_DONTFORK. MADV_DONTDUMP after MADV_DODUMP. Etc.. > > > But we already have MADV_HUGEPAGE, MADV_NOHUGEPAGE and prctl to > > enable/disable thp. Doesn't that sound little bit too much for a single > > feature to you? > > MADV_NOHUGEPAGE doesn't mean clearing the flag set with > MADV_HUGEPAGE. MADV_NOHUGEPAGE disables THP on the region if the > global sysfs "enabled" tune is set to "always". MADV_HUGEPAGE enables > THP if the global "enabled" sysfs tune is set to "madvise". The two > MADV_NOHUGEPAGE and MADV_HUGEPAGE are needed to leverage the three-way > setting of "never" "madvise" "always" of the global tune. > > The "madvise" global tune exists if you want to save RAM and you don't > care much about performance but still allowing apps like QEMU where no > memory is lost by enabling THP, to use THP. > > There's no way to clear either of those two flags and bring back the > default behavior of the global sysfs tune, so it's not redundant at > the very least. Yes I am not a huge fan of the current MADV*HUGEPAGE semantic but I would really like to see a strong usecase for adding another command on top. From what Mike said a global disable THP for the whole process while the post-copy is in progress is a better solution anyway. -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
