Re: [PATCH mm v3 29/38] kasan, vmalloc: add vmalloc tagging for HW_TAGS

[Andrey Konovalov]

On Tue, Dec 14, 2021 at 8:56 PM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Mon, Dec 13, 2021 at 10:54PM +0100, andrey.konovalov@xxxxxxxxx wrote:
> [...]
> >
> > +     /*
> > +      * Skip page_alloc poisoning and zeroing for pages backing VM_ALLOC
> > +      * mappings. Only effective in HW_TAGS mode.
> > +      */
> > +     gfp &= __GFP_SKIP_KASAN_UNPOISON & __GFP_SKIP_ZERO;
>
> This will turn gfp == 0 always. Should it have been
>
>         gfp |= __GFP_SKIP_KASAN_UNPOISON | __GFP_SKIP_ZERO

Oh, this is bad. Thanks for noticing! Will fix in v4.

> Also, not sure it matters, but on non-KASAN builds, this will now always
> generate an extra instruction. You could conditionally define GFP_SKIP*
> only in the KASAN modes that need them, otherwise they become 0, so the
> compiler optimizes this out. (Although I think it does does complicate
> GFP_SHIFT a little?)

I can implement this, but I don't think a single extra instruction per
vmalloc() matters.