On Thu, Jun 07, 2007 at 07:53:08AM -0400, Robert P. J. Day wrote: > > > > 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. :-)
Cache aliases. When the same page of physical memory is mapped twice to user space, let's say at address addr and addr + PAGE_SIZE this is normally harmless although wasteful on processors with virtually indexed caches as long as the page is mapped read-only such as in case of ZERO_PAGE. If the same thing happens with a writable page there is the chance of severe data corruption. Some members of the R4000 family are now trying to be helpful by throwing the kernel a "virtual coherency" exception. The bad news about this exception is there might be thousands (the theoretical worst case would be millions) of it in a single second, so servicing can be very expensive. For the ZERO page this can be avoided by using several pages mapped in a way such that their addresses don't conflict. Ralf - 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/
