On Tue, May 27, 2014 at 03:11:36PM +0400, Konstantin Khlebnikov wrote: > On Tue, May 27, 2014 at 2:54 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Tue, May 27, 2014 at 12:29:09PM +0200, Peter Zijlstra wrote: > >> On Tue, May 27, 2014 at 12:49:08AM +0400, Konstantin Khlebnikov wrote: > >> > Another suggestion. VM_RESERVED is stronger than VM_LOCKED and extends > >> > its functionality. > >> > Maybe it's easier to add VM_DONTMIGRATE and use it together with VM_LOCKED. > >> > This will make accounting easier. No? > >> > >> I prefer the PINNED name because the not being able to migrate is only > >> one of the desired effects of it, not the primary effect. We're really > >> looking to keep physical pages in place and preserve mappings. > > Ah, I just mixed it up. > > >> > >> The -rt people for example really want to avoid faults (even minor > >> faults), and DONTMIGRATE would still allow unmapping. > >> > >> Maybe always setting VM_PINNED and VM_LOCKED together is easier, I > >> hadn't considered that. The first thing that came to mind is that that > >> might make the fork() semantics difficult, but maybe it works out. > >> > >> And while we're on the subject, my patch preserves PINNED over fork() > >> but maybe we don't actually need that either. > > > > So pinned_vm is userspace exposed, which means we have to maintain the > > individual counts, and doing the fully orthogonal accounting is 'easier' > > than trying to get the boundary cases right. > > > > That is, if we have a program that does mlockall() and then does the IB > > ioctl() to 'pin' a region, we'd have to make mm_mpin() do munlock() > > after it splits the vma, and then do the pinned accounting. > > > > Also, we'll have lost the LOCKED state and unless MCL_FUTURE was used, > > we don't know what to restore the vma to on mm_munpin(). > > > > So while the accounting looks tricky, it has simpler semantics. > > What if VM_PINNED will require VM_LOCKED? > I.e. user must mlock it before pining and cannot munlock vma while it's pinned. So I don't like restrictions like that if its at all possible to avoid -- and in this case, I already wrote the code and its not _that_ complicated. But also; that would mean that we'd either have to make mm_mpin() do the mlock unconditionally (which rather defeats the purpose) or break userspace assumptions. I'm fairly sure the IB ioctl() don't require the memory to be mlocked. -- 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