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On 19/03/2026 12:09 pm, Ryan Roberts wrote: > On 19/03/2026 11:49, Muhammad Usama Anjum wrote: >> The stack pointer always uses the match-all tag, so MTE never checks >> tags on stack accesses. Tagging stack memory on every thread creation >> is pure overhead. >> >> - Pass __GFP_SKIP_KASAN in gfp_mask for vmalloc-backed stacks so the >> vmalloc path skips HW tag setup (see previous patch). >> - For the cached VMAP reuse path, skip kasan_unpoison_range() when HW >> tags are enabled since the memory will only be accessed through the >> match-all tagged SP. >> - For the normal page allocator path, pass __GFP_SKIP_KASAN directly >> to the page allocator. >> >> Signed-off-by: Muhammad Usama Anjum >> --- >> kernel/fork.c | 8 +++++--- >> 1 file changed, 5 insertions(+), 3 deletions(-) >> >> diff --git a/kernel/fork.c b/kernel/fork.c >> index bb0c2613a5604..2baf4db39b5a4 100644 >> --- a/kernel/fork.c >> +++ b/kernel/fork.c >> @@ -345,7 +345,8 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) >> } >> >> /* Reset stack metadata. */ >> - kasan_unpoison_range(vm_area->addr, THREAD_SIZE); >> + if (!kasan_hw_tags_enabled()) >> + kasan_unpoison_range(vm_area->addr, THREAD_SIZE); >> >> stack = kasan_reset_tag(vm_area->addr); >> >> @@ -358,7 +359,7 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) >> } >> >> stack = __vmalloc_node(THREAD_SIZE, THREAD_ALIGN, >> - GFP_VMAP_STACK, >> + GFP_VMAP_STACK | __GFP_SKIP_KASAN, > > Perhaps cleaner to include __GFP_SKIP_KASAN in GFP_VMAP_STACK ? Yes, it would be much better and the correct way. I'll add it in the next version. > >> node, __builtin_return_address(0)); >> if (!stack) >> return -ENOMEM; >> @@ -410,7 +411,8 @@ static void thread_stack_delayed_free(struct task_struct *tsk) >> >> static int alloc_thread_stack_node(struct task_struct *tsk, int node) >> { >> - struct page *page = alloc_pages_node(node, THREADINFO_GFP, >> + struct page *page = alloc_pages_node(node, >> + THREADINFO_GFP | __GFP_SKIP_KASAN, > > I think there are some other places that could benefit from __GFP_SKIP_KASAN; > see arm64's arch_alloc_vmap_stack(), which allocates stacks for efi, irq and > sdei. I think these are allocated at boot, so not really performance sensitive, > but we might as well be consistent? > > You've also missed the alloc_thread_stack_node() implementation for !VMAP when > PAGE_SIZE > STACK_SIZE. > > All of these sites use THREADINFO_GFP so perhaps it is better to just define > THREADINFO_GFP to include __GFP_SKIP_KASAN ? Yes, it'll be the straight forward and clean approach. I'll update. > > Thanks, > Ryan > > >> THREAD_SIZE_ORDER); >> >> if (likely(page)) { >