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Date: Tue, 27 May 2014 15:05:09 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Konstantin Khlebnikov <koct9i@...il.com> Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Christoph Lameter <cl@...ux.com>, Thomas Gleixner <tglx@...utronix.de>, Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>, Roland Dreier <roland@...nel.org>, Sean Hefty <sean.hefty@...el.com>, Hal Rosenstock <hal.rosenstock@...il.com>, Mike Marciniszyn <infinipath@...el.com> Subject: Re: [RFC][PATCH 0/5] VM_PINNED 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@...radead.org> 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 from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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