On Thu, Jun 18, 2020 at 11:58:47AM -0400, Waiman Long 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. > > Signed-off-by: Waiman Long <[email protected]> > --- > include/linux/sched/mm.h | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h > index 480a4d1b7dd8..09e01909d192 100644 > --- 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 = current->flags;
Per the above, I would've expected a READ_ONCE() there, otherwise the compiler can still re-load.
