On Thu, Sep 15, 2022 at 10:58 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, 15 Sep 2022 17:04:01 +0200 Alexander Potapenko <glider@xxxxxxxxxx> wrote: > > > KMSAN metadata for adjacent physical pages may not be adjacent, > > therefore accessing such pages together may lead to metadata > > corruption. > > We disable merging pages in biovec to prevent such corruptions. > > > > ... > > > > --- a/block/blk.h > > +++ b/block/blk.h > > @@ -88,6 +88,13 @@ static inline bool biovec_phys_mergeable(struct request_queue *q, > > phys_addr_t addr1 = page_to_phys(vec1->bv_page) + vec1->bv_offset; > > phys_addr_t addr2 = page_to_phys(vec2->bv_page) + vec2->bv_offset; > > > > + /* > > + * Merging adjacent physical pages may not work correctly under KMSAN > > + * if their metadata pages aren't adjacent. Just disable merging. > > + */ > > + if (IS_ENABLED(CONFIG_KMSAN)) > > + return false; > > + > > if (addr1 + vec1->bv_len != addr2) > > return false; > > if (xen_domain() && !xen_biovec_phys_mergeable(vec1, vec2->bv_page)) > > What are the runtime effects of this? In other words, how much > slowdown is this likely to cause in a reasonable worst-case? To be honest, I have no idea. KMSAN already introduces a lot of runtime overhead to every memory access, it's unlikely that disabling some filesystem optimization will add anything on top of that. Anyway, KMSAN is a debugging tool that is not supposed to be used in production (there's a big boot-time warning about that now :) ) -- Alexander Potapenko Software Engineer Google Germany GmbH Erika-Mann-Straße, 33 80636 München Geschäftsführer: Paul Manicle, Liana Sebastian Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg
