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. Thanks, Fabio
