On Thu, 7 Jun 2007, Satyam Sharma wrote: > On 6/7/07, Robert P. J. Day <[EMAIL PROTECTED]> wrote: > > On Thu, 7 Jun 2007, Nick Piggin wrote: > > > > > Robert P. J. Day wrote: > > > > probably making a fool of myself here, but what is the purpose of > > > > that single argument to the macro "ZERO_PAGE"? > > > > > > > > $ grep -r "define ZERO_PAGE" include > > > > include/asm-frv/pgtable.h:#define ZERO_PAGE(vaddr) ({ BUG(); NULL; > > }) > > > > include/asm-frv/pgtable.h:#define ZERO_PAGE(vaddr) > > > > virt_to_page(empty_zero_page) > > > > include/asm-v850/pgtable.h:#define ZERO_PAGE(vaddr) ((void > > *)0x87654321) > > > > include/asm-mips/pgtable.h:#define ZERO_PAGE(vaddr) \ > > > > include/asm-blackfin/pgtable.h:#define ZERO_PAGE(vaddr) > > (virt_to_page(0)) > > > > include/asm-parisc/pgtable.h:#define ZERO_PAGE(vaddr) > > > > (virt_to_page(empty_zero_page)) > > > > include/asm-alpha/pgtable.h:#define ZERO_PAGE(vaddr) > > > > (virt_to_page(ZERO_PGE)) > > > > ... > > > > > > > > AFAICT, there are no definitions of that macro that actually use > > > > that argument. is that some kind of historical cruft? > > > > > > MIPS? > > > > argh. that would be the *one* definition whose output got chopped > > because of line continuation, and it would be only one that actually > > uses the argument: > > > > #define ZERO_PAGE(vaddr) \ > > (virt_to_page((void *)(empty_zero_page + (((unsigned long)(vaddr)) & > > zero_page_mask)))) > > > > > > but it still leaves the question -- if ZERO_PAGE is meant to represent > > a single, global shared page that is always zero, why would it *ever* > > need to take an argument? and what's so special about MIPS that it > > differs from all the rest? > > The comment above empty_zero_page and zero_page_mask > declarations at arch/mips/mm/init.c:508 sheds light on this ...
well, it *sort of* does. at line 64 of that file: /* * We have up to 8 empty zeroed pages so we can map one of the right colour * when needed. This is necessary only on R4000 / R4400 SC and MC versions * where we have to avoid VCED / VECI exceptions for good performance at * any price. Since page is never written to after the initialization we * don't have to care about aliases on other CPUs. */ although it's not clear where in the source tree are the invocations that would actually make a difference to a MIPS system, which is why i've CC'ed ralf on this. i'm sure he can clear this up. :-) rday -- ======================================================================== Robert P. J. Day Linux Consulting, Training and Annoying Kernel Pedantry Waterloo, Ontario, CANADA http://fsdev.net/wiki/index.php?title=Main_Page ======================================================================== - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/