kexec: replace kmap() with kmap_local_page()

kmap() is 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 its use in kexec_core.c is safe everywhere, it should be preferred.

Therefore, replace kmap() with kmap_local_page() in kexec_core.c.

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

Link: https://lkml.kernel.org/r/20220821182519.9483-1-fmdefrancesco@gmail.com
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 5ca4d40c9ec133d06eb4d94a6b9b3867c2b40192..ca2743f9c634e4c94b4f4f30c19cd33549fb4cdf 100644 (file)
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -809,7 +809,7 @@ static int kimage_load_normal_segment(struct kimage *image,
                if (result < 0)
                        goto out;
 
-               ptr = kmap(page);
+               ptr = kmap_local_page(page);
                /* Start with a clear page */
                clear_page(ptr);
                ptr += maddr & ~PAGE_MASK;
@@ -822,7 +822,7 @@ static int kimage_load_normal_segment(struct kimage *image,
                        memcpy(ptr, kbuf, uchunk);
                else
                        result = copy_from_user(ptr, buf, uchunk);
-               kunmap(page);
+               kunmap_local(ptr);
                if (result) {
                        result = -EFAULT;
                        goto out;
@@ -873,7 +873,7 @@ static int kimage_load_crash_segment(struct kimage *image,
                        goto out;
                }
                arch_kexec_post_alloc_pages(page_address(page), 1, 0);
-               ptr = kmap(page);
+               ptr = kmap_local_page(page);
                ptr += maddr & ~PAGE_MASK;
                mchunk = min_t(size_t, mbytes,
                                PAGE_SIZE - (maddr & ~PAGE_MASK));
@@ -889,7 +889,7 @@ static int kimage_load_crash_segment(struct kimage *image,
                else
                        result = copy_from_user(ptr, buf, uchunk);
                kexec_flush_icache_page(page);
-               kunmap(page);
+               kunmap_local(ptr);
                arch_kexec_pre_free_pages(page_address(page), 1);
                if (result) {
                        result = -EFAULT;