On Wed, Nov 9, 2016 at 2:56 AM, Mark Rutland <mark.rutl...@arm.com> wrote: > On Tue, Nov 08, 2016 at 02:09:27PM -0800, Dmitry Vyukov wrote: >> On Tue, Nov 8, 2016 at 11:03 AM, Mark Rutland <mark.rutl...@arm.com> wrote: >> > When KASAN is selected, we allocate shadow for the whole vmalloc area, >> > using common zero pte, pmd, pud tables. Walking over these in the ptdump >> > code takes a *very* long time (I've seen up to 15 minutes with >> > KASAN_OUTLINE enabled). For DEBUG_WX [3], this means boot hangs for that >> > long, too. > > [...] > >> I've seen the same iteration slowness problem on x86 with >> CONFIG_DEBUG_RODATA which walks all pages. The is about 1 minute, but >> it is enough to trigger rcu stall warning. > > Interesting; do you know where that happens? I can't spot any obvious > case where we'd have to walk all the page tables for DEBUG_RODATA.
As far as I remember it was this path: mark_readonly in main.c -> mark_rodata_ro -> debug_checkwx -> ptdump_walk_pgd_level_checkwx -> ptdump_walk_pgd_level_core. >> The zero pud and vmalloc-ed stacks looks like different problems. >> To overcome the slowness we could map zero shadow for vmalloc area lazily. >> However for vmalloc-ed stacks we need to map actual memory, because >> stack instrumentation will read/write into the shadow. > > Sure. The point I was trying to make is that there' be fewer page tables > to walk (unless the vmalloc area was exhausted), assuming we also lazily > mapped the common zero shadow for the vmalloc area. > >> One downside here is that vmalloc shadow can be as large as 1:1 (if we >> allocate 1 page in vmalloc area we need to allocate 1 page for >> shadow). > > I thought per prior discussion we'd only need to allocate new pages for > the stacks in the vmalloc region, and we could re-use the zero pages? We can't reuse zero ro pages for stacks, because stack instrumentation writes to stack shadow. When we have a large continuous range of memory, shadow for it is 1/8th. However, if we have a separate page, we will need to map whole page of shadow for it, i.e. 1:1 shadow overhead. > ... or are you trying to quantify the cost of the page tables? > >> Re slowness: could we just skip the KASAN zero puds (the top level) >> while walking? Can they be interesting for anybody? > > They're interesting for the ptdump case (which allows privileged users > to dump the tables via /sys/kernel/debug/kernel_page_tables). I've seen > 25+ minute hangs there. > >> We can just pretend that they are not there. Looks like a trivial >> solution for the problem at hand. > > For the boot time hang it's option. Though I'd prefer that the sanity > checks applied to all of tables, shadow regions included. > > Thanks, > Mark. > > -- > You received this message because you are subscribed to the Google Groups > "kasan-dev" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to kasan-dev+unsubscr...@googlegroups.com. > To post to this group, send email to kasan-...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/kasan-dev/20161109105624.GA17020%40leverpostej. > For more options, visit https://groups.google.com/d/optout.
