On Wed, Feb 17, 2021 at 09:59:24PM +0100, Andrey Konovalov wrote: > During boot, all non-reserved memblock memory is exposed to the buddy > allocator. Poisoning all that memory with KASAN lengthens boot time, > especially on systems with large amount of RAM. This patch makes > page_alloc to not call kasan_free_pages() on all new memory. > > __free_pages_core() is used when exposing fresh memory during system > boot and when onlining memory during hotplug. This patch adds a new > FPI_SKIP_KASAN_POISON flag and passes it to __free_pages_ok() through > free_pages_prepare() from __free_pages_core(). > > This has little impact on KASAN memory tracking. > > Assuming that there are no references to newly exposed pages before they > are ever allocated, there won't be any intended (but buggy) accesses to > that memory that KASAN would normally detect. > > However, with this patch, KASAN stops detecting wild and large > out-of-bounds accesses that happen to land on a fresh memory page that > was never allocated. This is taken as an acceptable trade-off. > > All memory allocated normally when the boot is over keeps getting > poisoned as usual. > > Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> The approach looks fine to me. If you don't like the trade-off, I think you could still leave the kasan poisoning in if CONFIG_DEBUG_KERNEL. Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx> Just curious, have you noticed any issue booting a KASAN_SW_TAGS-enabled kernel on a system with sufficiently large RAM? Is the boot slow-down significant? For MTE, we could look at optimising the poisoning code for page size to use STGM or DC GZVA but I don't think we can make it unnoticeable for large systems (especially with DC GZVA, that's like zeroing the whole RAM at boot). -- Catalin
