On May 31, 2017 3:08:22 PM GMT+03:00, Michal Hocko <mhocko@xxxxxxxxxx> wrote: >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. That depends on the application access pattern in the period between the pre-dump is finished and the application is frozen. If the accesses are random enough, the dirty pages that would be post copied could get spread all over the address space. >> > 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? That would be helpful :) Still, the problem here is more with the naming that with the action. If it was called MADV_DEFAULT_READ or something, it would be fine, wouldn't it? >> 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. Well, another command makes the semantic a bit better, IMHO... > From what Mike said a global disable THP for the whole process >while the post-copy is in progress is a better solution anyway. For the CRIU usecase, disabling THP for a while and re-enabling it back will do the trick, provided VMAs flags are not affected, like in the patch you've sent. Moreover, we may even get away with ioctl(UFFDIO_COPY) if it's overhead shows to be negligible. Still, I believe that MADV_RESET_HUGEPAGE (or some better named) command has the value on its own. -- Sincerely yours, Mike. -- 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
