On 01/25/2016 11:07 AM, Christian Borntraeger wrote: > On 01/25/2016 11:02 AM, Heiko Carstens wrote: >> On Mon, Jan 25, 2016 at 10:45:50AM +0100, Christian Borntraeger wrote: >>>>> + By default this option will be almost for free and can be activated >>>>> + in distribution kernels. The overhead and the debugging can be enabled >>>>> + by DEBUG_PAGEALLOC_ENABLE_DEFAULT or the debug_pagealloc command line >>>>> + parameter. >>>> >>>> Sorry, but it's not almost for free and should not be used by distribution >>>> kernels. If we have DEBUG_PAGEALLOC enabled, at least on s390 we will not >>>> make use of 2GB and 1MB pagetable entries for the identy mapping anymore. >>>> Instead we will only use 4K mappings. >>> >>> Hmmm, can we change these code areas to use debug_pagealloc_enabled? I guess >>> this evaluated too late? >> >> Yes, that should be possible. "debug_pagealloc" is an early_param, which >> will be evaluated before we call paging_init() (both in >> arch/s390/kernel/setup.c). >> >> So it looks like this can be trivially changed. (replace the ifdefs in >> arch/s390/mm/vmem.c with debug_pagealloc_enabled()). >> >>>> I assume this is true for all architectures since freeing pages can happen >>>> in any context and therefore we can't allocate memory in order to split >>>> page tables. >>>> >>>> So enabling this will cost memory and put more pressure on the TLB. >>> >>> So I will change the description and drop the "if unsure" statement. >> >> Well, given that we can change it like above... I don't care anymore ;) > > Ok, I will give it a try, and come back with a rewording or an s390 patch. I have a patch for x86 and s390. powerpc should also be possible. Now it seems that sparc already defines the TSB very early in head.S. Unless we find a solution for sparc to use debug_pagealloc_enabled() I will modify the patch description and resend the patch. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
