On Wed, Mar 8, 2023 at 5:46 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > > On Tue, 28 Feb 2023 16:35:44 -0800 Peter Collingbourne <pcc@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx> # 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
