On Sat, Aug 13, 2022 at 03:36:53PM +0200, Fabio M. De Francesco wrote: > On mercoledì 3 agosto 2022 20:28:56 CEST Fabio M. De Francesco wrote: > > The use of kmap() and kmap_atomic() are being deprecated in favor of > > kmap_local_page(). > > > > There are two main problems with kmap(): (1) It comes with an overhead as > > mapping space is restricted and protected by a global lock for > > synchronization and (2) it also requires global TLB invalidation when the > > kmap’s pool wraps and it might block when the mapping space is fully > > utilized until a slot becomes available. > > > > With kmap_local_page() the mappings are per thread, CPU local, can take > > page faults, and can be called from any context (including interrupts). > > It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore, > > the tasks can be preempted and, when they are scheduled to run again, the > > kernel virtual addresses are restored and are still valid. > > > > Since the use of kmap_local_page() in exec.c is safe, it should be > > preferred everywhere in exec.c. > > > > As said, since kmap_local_page() can be also called from atomic context, > > and since remove_arg_zero() doesn't (and shouldn't ever) rely on an > > implicit preempt_disable(), this function can also safely replace > > kmap_atomic(). > > > > Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in > > fs/exec.c. > > > > Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel > > with HIGHMEM64GB enabled. > > > > Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> > > Suggested-by: Ira Weiny <ira.weiny@xxxxxxxxx> > > Reviewed-by: Ira Weiny <ira.weiny@xxxxxxxxx> > > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> > > --- > > > > v1->v2: Added more information to the commit log to address some > > objections expressed by Eric W. Biederman[1] in reply to v1. No changes > > have been made to the code. Forwarded a tag from Ira Weiny (thanks!). > > > > [1] > > https://lore.kernel.org/lkml/8735fmqcfz.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > > > fs/exec.c | 14 +++++++------- > > 1 file changed, 7 insertions(+), 7 deletions(-) > > > > Hi Kees, > > After that thread about the report from Syzbot, and the subsequent discussion, > I noticed that you didn't yet take this other patch for exec.c. > > I suppose that the two patches would better go out together. So I'm writing > for sending a gentle ping. > > As I said, no changes have been made to the code with respect to v1 (which I > submitted in June). However, later I thought that adding more information > might have helped reviewers and maintainers to better understand the why of > this patch. Oops, thanks for the ping. I'll pull this now. -- Kees Cook
