* Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > On Tue, Oct 24, 2017 at 02:47:41PM +0200, Ingo Molnar wrote: > > > > > > > > Making a variable that 'looks' like a constant macro dynamic in a rare Kconfig > > > > > > > > scenario is asking for trouble. > > > > > > > > > > > > > > We expect boot-time page mode switching to be enabled in kernel of next > > > > > > > generation enterprise distros. It shoudn't be that rare. > > > > > > > > > > > > My point remains even with not-so-rare Kconfig dependency. > > > > > > > > > > I don't follow how introducing new variable that depends on Kconfig option > > > > > would help with the situation. > > > > > > > > A new, properly named variable or function (max_physmem_bits or > > > > max_physmem_bits()) that is not all uppercase would make it abundantly clear that > > > > it is not a constant but a runtime value. > > > > > > Would we need to rename every uppercase macros that would depend on > > > max_physmem_bits()? Like MAXMEM. > > > > MAXMEM isn't used in too many places either - what's the total impact of it? > > The impact is not very small. The tree of macros dependent on > MAX_PHYSMEM_BITS: > > MAX_PHYSMEM_BITS > MAXMEM > KEXEC_SOURCE_MEMORY_LIMIT > KEXEC_DESTINATION_MEMORY_LIMIT > KEXEC_CONTROL_MEMORY_LIMIT > SECTIONS_SHIFT > ZONEID_SHIFT > ZONEID_PGSHIFT > ZONEID_MASK > > The total number of users of them is not large. It's doable. But I expect > it to be somewhat ugly, since we're partly in generic code and it would > require some kind of compatibility layer for other archtectures. > > Do you want me to rename them all? Yeah, I think these former constants should be organized better. Here's their usage frequency: triton:~/tip> for N in MAX_PHYSMEM_BITS MAXMEM KEXEC_SOURCE_MEMORY_LIMIT \ KEXEC_DESTINATION_MEMORY_LIMIT KEXEC_CONTROL_MEMORY_LIMIT SECTIONS_SHIFT \ ZONEID_SHIFT ZONEID_PGSHIFT ZONEID_MASK; do printf " %-40s: " $N; git grep -w $N | grep -vE 'define| \* ' | wc -l; done MAX_PHYSMEM_BITS : 10 MAXMEM : 5 KEXEC_SOURCE_MEMORY_LIMIT : 2 KEXEC_DESTINATION_MEMORY_LIMIT : 2 KEXEC_CONTROL_MEMORY_LIMIT : 2 SECTIONS_SHIFT : 2 ZONEID_SHIFT : 1 ZONEID_PGSHIFT : 1 ZONEID_MASK : 1 So it's not too bad to clean up, I think. How about something like this: machine.physmem.max_bytes /* ex MAXMEM */ machine.physmem.max_bits /* bit count of the highest in-use physical address */ machine.physmem.zones.id_shift /* ZONEID_SHIFT */ machine.physmem.zones.pg_shift /* ZONEID_PGSHIFT */ machine.physmem.zones.id_mask /* ZONEID_MASK */ machine.kexec.physmem_bytes_src /* KEXEC_SOURCE_MEMORY_LIMIT */ machine.kexec.physmem_bytes_dst /* KEXEC_DESTINATION_MEMORY_LIMIT */ ( With perhaps 'physmem' being an alias to '&machine->physmem', so that physmem->max_bytes and physmem->max_bits would be a natural thing to write. ) I'd suggest doing this in a finegrained fashion, one step at a time, introducing 'struct machine' and 'struct physmem' and extending it gradually with new fields. To re-discuss the virt_addr_valid() concern you raised: > > For instance, virt_addr_valid() depends indirectly on it: > > > > virt_addr_valid() > > __virt_addr_valid() > > phys_addr_valid() > > boot_cpu_data.x86_phys_bits (initialized with MAX_PHYSMEM_BITS) > > > > virt_addr_valid() is used in things like implementation /dev/kmem. > > > > To me it's far more risky than occasional build breakage for > > CONFIG_X86_5LEVEL=y. > > So why do we have two variables here, one boot_cpu_data.x86_phys_bits and the > other MAX_PHYSMEM_BITS - both set once during boot? So it's still unclear to me why virt_addr_valid() would be a problem: this function could probably (in a separate patch) use physmem->max_bits, which would make it more secure than using even a dynamic MAX_PHYSMEM_BITS: it would detect any physical addresses that are beyond the recognized maximum range. I.e. all this would result in further improvements. Thanks, Ingo -- 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>
