From afd584398b51eba12d0ffa4e0e0a493323993103 Mon Sep 17 00:00:00 2001 From: ZhangPeng Date: Wed, 27 Mar 2024 17:08:35 +0800 Subject: [PATCH] userfaultfd: early return in dup_userfaultfd() When vma->vm_userfaultfd_ctx.ctx is NULL, vma->vm_flags should have cleared __VM_UFFD_FLAGS. Therefore, there is no need to down_write or clear the flag, which will affect fork performance. Fix this by returning early if octx is NULL in dup_userfaultfd(). By applying this patch we can get a 1.3% performance improvement for lmbench fork_prot. Results are as follows: base early return Process fork+exit: 419.1106 413.4804 Link: https://lkml.kernel.org/r/20240327090835.3232629-1-zhangpeng362@huawei.com Signed-off-by: ZhangPeng Reviewed-by: Peter Xu Cc: Axel Rasmussen Cc: David Hildenbrand Cc: Kefeng Wang Cc: Liam R. Howlett Cc: Lokesh Gidra Cc: Nanyong Sun Cc: Suren Baghdasaryan Signed-off-by: Andrew Morton --- fs/userfaultfd.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 60dcfafdc11a..3e6ddda6f159 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -657,7 +657,10 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs) struct userfaultfd_fork_ctx *fctx; octx = vma->vm_userfaultfd_ctx.ctx; - if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) { + if (!octx) + return 0; + + if (!(octx->features & UFFD_FEATURE_EVENT_FORK)) { vma_start_write(vma); vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX; userfaultfd_set_vm_flags(vma, vma->vm_flags & ~__VM_UFFD_FLAGS);