linux-kernel - Re: [PATCH 08/31] kasan, page_alloc: refactor init checks in post_alloc

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Date:   Thu, 16 Dec 2021 11:59:38 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Andrey Konovalov <andreyknvl@...il.com>
Cc:     andrey.konovalov@...ux.dev, Marco Elver <elver@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Peter Collingbourne <pcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH 08/31] kasan, page_alloc: refactor init checks in post_alloc_hook

On Mon, Dec 6, 2021 at 10:09 PM Andrey Konovalov <andreyknvl@...il.com> wrote:
>
> On Thu, Dec 2, 2021 at 5:14 PM Alexander Potapenko <glider@...gle.com> wrote:
> >
> > On Tue, Nov 30, 2021 at 10:41 PM <andrey.konovalov@...ux.dev> wrote:
> > >
> > > From: Andrey Konovalov <andreyknvl@...gle.com>
> > >
> > > This patch separates code for zeroing memory from the code clearing tags
> > > in post_alloc_hook().
> > >
> > > This patch is not useful by itself but makes the simplifications in
> > > the following patches easier to follow.
> > >
> > > This patch does no functional changes.
> > >
> > > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> > > ---
> > >  mm/page_alloc.c | 18 ++++++++++--------
> > >  1 file changed, 10 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > > index 2ada09a58e4b..0561cdafce36 100644
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -2406,19 +2406,21 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
> > >                 kasan_alloc_pages(page, order, gfp_flags);
> > >         } else {
> > >                 bool init = !want_init_on_free() && want_init_on_alloc(gfp_flags);
> > > +               bool init_tags = init && (gfp_flags & __GFP_ZEROTAGS);
> > >
> > >                 kasan_unpoison_pages(page, order, init);
> > >
> > > -               if (init) {
> > > -                       if (gfp_flags & __GFP_ZEROTAGS) {
> > > -                               int i;
> > > +               if (init_tags) {
> > > +                       int i;
> > >
> > > -                               for (i = 0; i < 1 << order; i++)
> > > -                                       tag_clear_highpage(page + i);
> > > -                       } else {
> > > -                               kernel_init_free_pages(page, 1 << order);
> > > -                       }
> > > +                       for (i = 0; i < 1 << order; i++)
> > > +                               tag_clear_highpage(page + i);
> > > +
> > > +                       init = false;
> >
> > I find this a bit twisted and prone to breakages.
> > Maybe just check for (init && !init_tags) below?
>
> I did it this way deliberately. Check out the code after all the changes:
>
> https://github.com/xairy/linux/blob/up-kasan-vmalloc-tags-v1/mm/page_alloc.c#L2447
>
> It's possible to remove resetting the init variable by expanding the
> if (init) check listing all conditions under which init is currently
> reset, but that would essentially be duplicating the checks. I think
> resetting init is more clear.
>
> Please let me know what you think.

Ah, I see, so there are more cases in which you set init = false.
Fine then.