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From:   Andrey Konovalov <andreyknvl@gmail.com>
Date:   Mon, 19 Dec 2022 19:08:59 +0100
Message-ID: <CA+fCnZcDEV4hmeyLb6paTvR7Z3gjQOTJn_M9wTMN-cy+9DKUTw@mail.gmail.com>
Subject: Re: [PATCH v3] kasan: allow sampling page_alloc allocations for HW_TAGS
To:     Marco Elver <elver@google.com>
Cc:     andrey.konovalov@linux.dev,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        kasan-dev@googlegroups.com, Peter Collingbourne <pcc@google.com>,
        Evgenii Stepanov <eugenis@google.com>,
        Florian Mayer <fmayer@google.com>,
        Jann Horn <jannh@google.com>,
        Mark Brand <markbrand@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrey Konovalov <andreyknvl@google.com>
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On Mon, Dec 19, 2022 at 12:31 PM Marco Elver <elver@google.com> wrote:
>
> On a whole:
>
> Reviewed-by: Marco Elver <elver@google.com>
>
> This looks much better, given it'll automatically do the right thing
> without marking costly allocation sites.

Agreed, thank you for the suggestion!

> > +- ``kasan.page_alloc.sample.order=<minimum page order>`` specifies the minimum
> > +  order of allocations that are affected by sampling (default: ``3``).
> > +  Only applies when ``kasan.page_alloc.sample`` is set to a non-default value.
>
> "set to a value greater than 1"? The additional indirection through
> "non-default" seems unnecessary.

Will fix in v4.

> > +  This parameter is intended to allow sampling only large page_alloc
> > +  allocations, which is the biggest source of the performace overhead.
>
> s/performace/performance/

Will fix in v4.

> > --- a/mm/kasan/hw_tags.c
> > +++ b/mm/kasan/hw_tags.c
> > @@ -59,6 +59,24 @@ EXPORT_SYMBOL_GPL(kasan_mode);
> >  /* Whether to enable vmalloc tagging. */
> >  DEFINE_STATIC_KEY_TRUE(kasan_flag_vmalloc);
> >
> > +#define PAGE_ALLOC_SAMPLE_DEFAULT      1
> > +#define PAGE_ALLOC_SAMPLE_ORDER_DEFAULT        3
>
> Why not just set it to PAGE_ALLOC_COSTLY_ORDER?

I've been thinking about this, but technically PAGE_ALLOC_COSTLY_ORDER
is related to allocations that are costly to service due to
fragmentation/reclaim-related issues. We also don't rely on
PAGE_ALLOC_COSTLY_ORDER only, but also on SKB_FRAG_PAGE_ORDER. (I
guess some clean-up is possible wrt these constants: I suspect both
have the same value for the same reason. But I don't want to attempt
it with this patch. )

We could add a BUILD_BUG_ON that makes sure that all 3 constants are
the same. But then the only thing to do if one of them is changed is
to remove the BUG_ON, which doesn't seem very useful.

I'll leave the current implementation in v4.

Thank you, Marco!