On Tue, Sep 11, 2018 at 1:01 PM, Alexander Duyck <alexander.duyck@xxxxxxxxx> wrote: > On Tue, Sep 11, 2018 at 9:50 AM Dan Williams <dan.j.williams@xxxxxxxxx> wrote: >> >> On Mon, Sep 10, 2018 at 4:43 PM, Alexander Duyck >> <alexander.duyck@xxxxxxxxx> wrote: >> > From: Alexander Duyck <alexander.h.duyck@xxxxxxxxx> >> > >> > On systems with a large amount of memory it can take a significant amount >> > of time to initialize all of the page structs with the PAGE_POISON_PATTERN >> > value. I have seen it take over 2 minutes to initialize a system with >> > over 12GB of RAM. >> > >> > In order to work around the issue I had to disable CONFIG_DEBUG_VM and then >> > the boot time returned to something much more reasonable as the >> > arch_add_memory call completed in milliseconds versus seconds. However in >> > doing that I had to disable all of the other VM debugging on the system. >> > >> > In order to work around a kernel that might have CONFIG_DEBUG_VM enabled on >> > a system that has a large amount of memory I have added a new kernel >> > parameter named "page_init_poison" that can be set to "off" in order to >> > disable it. >> >> In anticipation of potentially more DEBUG_VM options wanting runtime >> control I'd propose creating a new "vm_debug=" option for this modeled >> after "slub_debug=" along with a CONFIG_DEBUG_VM_ON to turn on all >> options. >> >> That way there is more differentiation for debug cases like this that >> have significant performance impact when enabled. >> >> CONFIG_DEBUG_VM leaves optional debug capabilities disabled by default >> unless CONFIG_DEBUG_VM_ON is also set. > > Based on earlier discussions I would assume that CONFIG_DEBUG_VM would > imply CONFIG_DEBUG_VM_ON anyway since we don't want most of these > disabled by default. > > In my mind we should be looking at a selective "vm_debug_disable=" > instead of something that would be turning on features. Sorry, I missed those earlier discussions, so I won't push too hard if this has been hashed before. My proposal for opt-in is the fact that at least one known distribution kernel, Fedora, is shipping with CONFIG_DEBUG_VM=y. They also ship with CONFIG_SLUB, but not SLUB_DEBUG_ON. If we are going to picemeal enable some debug options to be runtime controlled I think we should go further to start clarifying the cheap vs the expensive checks and making the expensive checks opt-in in the same spirit of SLUB_DEBUG.
