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From:   Alexander Potapenko <glider@google.com>
Date:   Fri, 16 Sep 2022 11:12:44 +0200
Message-ID: <CAG_fn=WJZBK_xypJ-D7NPjGeaQ8c3fs8Ji+-j+=O=9neZjTUBw@mail.gmail.com>
Subject: Re: [PATCH v7 27/43] kmsan: disable physical page merging in biovec
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrey Konovalov <andreyknvl@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Christoph Hellwig <hch@lst.de>,
        Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Eric Biggers <ebiggers@kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Ilya Leoshkevich <iii@linux.ibm.com>,
        Ingo Molnar <mingo@redhat.com>, Jens Axboe <axboe@kernel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Matthew Wilcox <willy@infradead.org>,
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        Pekka Enberg <penberg@kernel.org>,
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        Vegard Nossum <vegard.nossum@oracle.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
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On Thu, Sep 15, 2022 at 10:58 PM Andrew Morton
<akpm@linux-foundation.org> wrote:
>
> On Thu, 15 Sep 2022 17:04:01 +0200 Alexander Potapenko <glider@google.com=
> 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 requ=
est_queue *q,
> >       phys_addr_t addr1 =3D page_to_phys(vec1->bv_page) + vec1->bv_offs=
et;
> >       phys_addr_t addr2 =3D page_to_phys(vec2->bv_page) + vec2->bv_offs=
et;
> >
> > +     /*
> > +      * Merging adjacent physical pages may not work correctly under K=
MSAN
> > +      * if their metadata pages aren't adjacent. Just disable merging.
> > +      */
> > +     if (IS_ENABLED(CONFIG_KMSAN))
> > +             return false;
> > +
> >       if (addr1 + vec1->bv_len !=3D addr2)
> >               return false;
> >       if (xen_domain() && !xen_biovec_phys_mergeable(vec1, vec2->bv_pag=
e))
>
> 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 :) )

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Alexander Potapenko
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