Re: [PATCH v7 27/43] kmsan: disable physical page merging in biovec

[Alexander Potapenko]

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

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