On Tue, Nov 13, 2012 at 11:21:20AM +0100, Ingo Molnar wrote: > > * Mel Gorman <mgorman@xxxxxxx> wrote: > > > Note: This patch started as "mm/mpol: Create special PROT_NONE > > infrastructure" and preserves the basic idea but steals *very* > > heavily from "autonuma: numa hinting page faults entry points" for > > the actual fault handlers without the migration parts. The end > > result is barely recognisable as either patch so all Signed-off > > and Reviewed-bys are dropped. If Peter, Ingo and Andrea are ok with > > this version, I will re-add the signed-offs-by to reflect the history. > > Most of the changes you had to do here relates to the earlier > decision to turn it all the NUMA protection fault demultiplexing > and setup code into a per arch facility. > Yes. > On one hand I'm 100% fine with making the decision to *use* the > new NUMA code per arch and explicitly opt-in - we already have > such a Kconfig switch in our tree already. The decision whether > to use any of this for an architecture must be considered and > tested carefully. > Agreed. > But given that most architectures will be just fine reusing the > already existing generic PROT_NONE machinery, the far better > approach is to do what we've been doing in generic kernel code > for the last 10 years: offer a default generic version, and then > to offer per arch hooks on a strict as-needed basis, if they > want or need to do something weird ... > If they are *not* fine with it, it's a large retrofit because the PROT_NONE machinery has been hard-coded throughout. It also requires that anyone looking at the fault paths must remember at almost all times that PROT_NONE can also mean PROT_NUMA and it depends on context. While that's fine right now, it'll be harder to maintain in the future. > So why fork away this logic into per arch code so early and > without explicit justification? It creates duplication artifacts > all around and makes porting to a new 'sane' architecture > harder. > I agree that the duplication artifacts was a mistake. I can fix that but feel that the naming is fine and we shouldn't hard-code that change_prot_none() can actually mean change_prot_numa() if called from the right place. > Also, if there *are* per architecture concerns then I'd very > much like to see that argued very explicitly, on a per arch > basis, as it occurs, not obscured through thick "just in case" > layers of abstraction ... > Once there is a generic pte_numa handler for example, an arch-specific overriding of it should raise a red flag for closer inspection. -- Mel Gorman SUSE Labs -- 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>