fs/ecryptfs: Use kmap_local_page() in copy_up_encrypted_with_header()

fs/ecryptfs: Use kmap_local_page() in copy_up_encrypted_with_header()

kmap_atomic() has been deprecated in favor of kmap_local_page().

Therefore, replace kmap_atomic() with kmap_local_page() in
ecryptfs_copy_up_encrypted_with_header().

kmap_atomic() is implemented like a kmap_local_page() which also
disables page-faults and preemption (the latter only in !PREEMPT_RT
kernels). The kernel virtual addresses returned by these two API are
only valid in the context of the callers (i.e., they cannot be handed to
other threads).

With kmap_local_page() the mappings are per thread and CPU local like
in kmap_atomic(); however, they can handle page-faults and can be called
from any context (including interrupts). The tasks that call
kmap_local_page() can be preempted and, when they are scheduled to run
again, the kernel virtual addresses are restored and are still valid.

In ecryptfs_copy_up_encrypted_with_header(), the block of code between
the mapping and un-mapping does not depend on the above-mentioned side
effects of kmap_aatomic(), so that the mere replacements of the old API
with the new one is all that is required (i.e., there is no need to
explicitly call pagefault_disable() and/or preempt_disable()).

Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

Cc: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Message-Id: <20230426172223.8896-4-fmdefrancesco@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>