On Thu, 18 Jun 2020 17:29:36 -0400 Waiman Long <[email protected]> wrote:
> The current_gfp_context() converts a number of PF_MEMALLOC_* per-process
> flags into the corresponding GFP_* flags for memory allocation. In
> that function, current->flags is accessed 3 times. That may lead to
> duplicated access of the same memory location.
>
> This is not usually a problem with minimal debug config options on as the
> compiler can optimize away the duplicated memory accesses. With most
> of the debug config options on, however, that may not be the case.
> For example, the x86-64 object size of the __need_fs_reclaim() in a
> debug kernel that calls current_gfp_context() was 309 bytes. With this
> patch applied, the object size is reduced to 202 bytes. This is a saving
> of 107 bytes and will probably be slightly faster too.
>
> ...
>
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -181,18 +181,20 @@ static inline bool in_vfork(struct task_struct *tsk)
> */
> static inline gfp_t current_gfp_context(gfp_t flags)
> {
> - if (unlikely(current->flags &
> + unsigned int pflags = READ_ONCE(current->flags);
Why use READ_ONCE() here?
> + if (unlikely(pflags &
> (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS |
> PF_MEMALLOC_NOCMA))) {
> /*
> * NOIO implies both NOIO and NOFS and it is a weaker context
> * so always make sure it makes precedence
> */
> - if (current->flags & PF_MEMALLOC_NOIO)
> + if (pflags & PF_MEMALLOC_NOIO)
> flags &= ~(__GFP_IO | __GFP_FS);
> - else if (current->flags & PF_MEMALLOC_NOFS)
> + else if (pflags & PF_MEMALLOC_NOFS)
> flags &= ~__GFP_FS;
> #ifdef CONFIG_CMA
> - if (current->flags & PF_MEMALLOC_NOCMA)
> + if (pflags & PF_MEMALLOC_NOCMA)
> flags &= ~__GFP_MOVABLE;
> #endif
> }
