On Wednesday, 5 of November 2008, Nigel Cunningham wrote: > Hi. > > On Mon, 2008-11-03 at 23:53 -0800, Dave Hansen wrote: > > On Tue, 2008-11-04 at 18:30 +1100, Nigel Cunningham wrote: > > > One other question, if I may. Would you please explain (or point me to > > > an explanation) of PHYS_PFN_OFFSET/ARCH_PFN_OFFSET? I've been dealing > > > occasionally with people wanting to have hibernation on arm, and I don't > > > really get the concept or the implementation (particularly when it comes > > > to trying to do the sort of iterating over zones and pfns that was being > > > discussed in previous messages in this thread. > > > > First of all, I think PHYS_PFN_OFFSET is truly an arch-dependent > > construct. It only appears in arm an avr32. I'll tell you only how > > ARCH_PFN_OFFSET looks to me. My guess is that those two arches need to > > reconcile themselves and start using ARCH_PFN_OFFSET instead. > > > > In the old days, we only had memory that started at physical address 0x0 > > and went up to some larger address. We allocated a mem_map[] of 'struct > > pages' in one big chunk, one for each address. mem_map[0] was for > > physical address 0x0 and mem_map[1] was for 0x1000, mem_map[2] was for > > 0x2000 and so on... > > > > If a machine didn't have a physical address 0x0, we allocated mem_map[] > > for it anyway and just wasted that entry. What ARCH_PFN_OFFSET does is > > let us bias the mem_map[] structure so that mem_map[0] does not > > represent 0x0. > > > > If ARCH_PFN_OFFSET is 1, then mem_map[0] actually represents the > > physical address 0x1000. If it is 2, then mem_map[0] represents > > physical addr 0x2000. ARCH_PFN_OFFSET means that the first physical > > address on the machine is at ARCH_PFN_OFFSET*PAGE_SIZE. We bias all > > lookups into the mem_map[] so that we don't waste space in it. There > > will never be a zone_start_pfn lower than ARCH_PFN_OFFSET, for instance. > > > > What does that mean for walking zones? Nothing. It only has meaning > > for how we allocate and do lookups into the mem_map[]. But, since > > everyone uses pfn_to_page() and friends, you don't ever see this. > > > > I'm curious why you think you need to be concerned with it. > > Sorry for the delay in replying. > > It's because I'm looking at old patches for arm support for TuxOnIce and > because of the way TuxOnIce records what pages need attention: > > My method of recording what needs doing is different to Rafael's. I use > per zone bitmaps (constructed out of order 0 allocations) and therefore > look at zone_start_pfn in calculating what bit within the zone needs to > be set/cleared/tested. Well, the mainline does pretty much the same at the moment, but the bitmaps are probably different. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
