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

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Message-ID: <CAG_fn=WJZBK_xypJ-D7NPjGeaQ8c3fs8Ji+-j+=O=9neZjTUBw@mail.gmail.com>
Date:   Fri, 16 Sep 2022 11:12:44 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 27/43] kmsan: disable physical page merging in biovec

On Thu, Sep 15, 2022 at 10:58 PM Andrew Morton
<akpm@...ux-foundation.org> wrote:
>
> On Thu, 15 Sep 2022 17:04:01 +0200 Alexander Potapenko <glider@...gle.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 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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