linux-kernel - [PATCH v3] kexec: Replace kmap() with kmap_local

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Date:   Wed,  3 Aug 2022 18:00:38 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Eric Biederman <ebiederm@...ssion.com>, kexec@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Baoquan He <bhe@...hat.com>,
        Ira Weiny <ira.weiny@...el.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Subject: [PATCH v3] 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.

Suggested-by: Ira Weiny <ira.weiny@...el.com>
Reviewed-by: Ira Weiny <ira.weiny@...el.com>
Acked-by: Baoquan He <bhe@...hat.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---

v2->v3: Add more information in the commit message as requested by Baoquan
He and forward two tags, respectively from Ira Weiny and Baoquan He (many
thanks to them two!).

v1->v2: A sentence of the commit message contained an error due to a
mistake in copy-pasting from a previous patch. Replace "aio.c" with
"kexec_core.c".

 kernel/kexec_core.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index 4d34c78334ce..6f98274765d4 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -814,7 +814,7 @@ 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;
@@ -827,7 +827,7 @@ 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;
@@ -878,7 +878,7 @@ 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));
@@ -894,7 +894,7 @@ 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;
-- 
2.37.1