On 04/02/2014 12:47 PM, Johannes Weiner wrote: > On Wed, Apr 02, 2014 at 12:01:00PM -0700, John Stultz wrote: >> On Wed, Apr 2, 2014 at 10:58 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: >>> On Wed, Apr 02, 2014 at 10:40:16AM -0700, John Stultz wrote: >>>> That point beside, I think the other problem with the page-cleaning >>>> volatility approach is that there are other awkward side effects. For >>>> example: Say an application marks a range as volatile. One page in the >>>> range is then purged. The application, due to a bug or otherwise, >>>> reads the volatile range. This causes the page to be zero-filled in, >>>> and the application silently uses the corrupted data (which isn't >>>> great). More problematic though, is that by faulting the page in, >>>> they've in effect lost the purge state for that page. When the >>>> application then goes to mark the range as non-volatile, all pages are >>>> present, so we'd return that no pages were purged. From an >>>> application perspective this is pretty ugly. >>>> >>>> Johannes: Any thoughts on this potential issue with your proposal? Am >>>> I missing something else? >>> No, this is accurate. However, I don't really see how this is >>> different than any other use-after-free bug. If you access malloc >>> memory after free(), you might receive a SIGSEGV, you might see random >>> data, you might corrupt somebody else's data. This certainly isn't >>> nice, but it's not exactly new behavior, is it? >> The part that troubles me is that I see the purged state as kernel >> data being corrupted by userland in this case. The kernel will tell >> userspace that no pages were purged, even though they were. Only >> because userspace made an errant read of a page, and got garbage data >> back. > That sounds overly dramatic to me. First of all, this data still > reflects accurately the actions of userspace in this situation. And > secondly, the kernel does not rely on this data to be meaningful from > a userspace perspective to function correctly. <insert dramatic-chipmunk video w/ text overlay "errant read corrupted volatile page purge state!!!!1"> Maybe you're right, but I feel this is the sort of thing application developers would be surprised and annoyed by. > It's really nothing but a use-after-free bug that has consequences for > no-one but the faulty application. The thing that IS new is that even > a read is enough to corrupt your data in this case. > > MADV_REVIVE could return 0 if all pages in the specified range were > present, -Esomething if otherwise. That would be semantically sound > even if userspace messes up. So its semantically more of just a combined mincore+dirty operation.. and nothing more? What are other folks thinking about this? Although I don't particularly like it, I probably could go along with Johannes' approach, forgoing SIGBUS for zero-fill and adapting the semantics that are in my mind a bit stranger. This would allow for ashmem-like style behavior w/ the additional write-clears-volatile-state and read-clears-purged-state constraints (which I don't think would be problematic for Android, but am not totally sure). But I do worry that these semantics are easier for kernel-mm-developers to grasp, but are much much harder for application developers to understand. Additionally unless we could really leave access-after-volatile as a total undefined behavior, this would lock us into O(page) behavior and would remove the possibility of O(log(ranges)) behavior Minchan and I were able to get (admittedly with more complicated code - but something I was hoping we'd be able to get back to after the base semantics and interface behavior was understood and merged). I since applications will have bugs and will access after volatile, we won't be able to get away with that sort of behavioral flexibility. thanks -john -- 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>