[PATCH v3 1/2] Revert "kasan: drop skip_kasan_poison variable in free_pages_prepare"

[Peter Collingbourne]

On Wed, Mar 8, 2023 at 5:46 PM Andrew Morton <akpm at linux-foundation.org> wrote:
>
> On Tue, 28 Feb 2023 16:35:44 -0800 Peter Collingbourne <pcc at google.com> wrote:
>
> > This reverts commit 487a32ec24be819e747af8c2ab0d5c515508086a.
> >
> > The should_skip_kasan_poison() function reads the PG_skip_kasan_poison
> > flag from page->flags. However, this line of code in free_pages_prepare():
> >
> > page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> >
> > clears most of page->flags, including PG_skip_kasan_poison, before calling
> > should_skip_kasan_poison(), which meant that it would never return true
> > as a result of the page flag being set. Therefore, fix the code to call
> > should_skip_kasan_poison() before clearing the flags, as we were doing
> > before the reverted patch.
>
> What are the user visible effects of this change?
>
> > Cc: <stable at vger.kernel.org> # 6.1
>
> Especially if it's cc:stable.

This fixes a measurable performance regression introduced in the
reverted commit, where munmap() takes longer than intended if HW tags
KASAN is supported and enabled at runtime. Without this patch, we see
a single-digit percentage performance regression in a particular
mmap()-heavy benchmark when enabling HW tags KASAN, and with the
patch, there is no statistically significant performance impact when
enabling HW tags KASAN.

That can be added as a paragraph to the end of my commit message, or I
can send a v4 if you prefer.

Peter