linux-kernel - [PATCH v2] lib: Replace kmap() with kmap_local

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Message-ID: <20230610175712.GA348514@sumitra.com>
Date:   Sat, 10 Jun 2023 10:57:12 -0700
From:   Sumitra Sharma <sumitraartsy@...il.com>
To:     Jérôme Glisse <jglisse@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>, Fabio <fmdefrancesco@...il.com>,
        Deepak R Varma <drv@...lo.com>
Subject: [PATCH v2] lib: Replace kmap() with kmap_local_page()

kmap() has been deprecated in favor of the kmap_local_page()
due to high cost, restricted mapping space, the overhead of
a global lock for synchronization, and making the process
sleep in the absence of free slots.

kmap_local_page() is faster than kmap() and offers thread-local
and CPU-local mappings, take pagefaults in a local kmap region
and preserves preemption by saving the mappings of outgoing
tasks and restoring those of the incoming one during a context
switch.

The mappings are kept thread local in the functions
“dmirror_do_read” and “dmirror_do_write” in test_hmm.c

Therefore, replace kmap() with kmap_local_page() and use
mempcy_from/to_page() to avoid open coding kmap_local_page()
+ memcpy() + kunmap_local().

Remove the unused variable “tmp”.


Suggested-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Signed-off-by: Sumitra Sharma <sumitraartsy@...il.com>
---

Changes in v2: 
	- Change commit subject and description.
	- Remove unnecessary type casting (char *).


 lib/test_hmm.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/lib/test_hmm.c b/lib/test_hmm.c
index 67e6f83fe0f8..717dcb830127 100644
--- a/lib/test_hmm.c
+++ b/lib/test_hmm.c
@@ -368,16 +368,13 @@ static int dmirror_do_read(struct dmirror *dmirror, unsigned long start,
 	for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++) {
 		void *entry;
 		struct page *page;
-		void *tmp;
 
 		entry = xa_load(&dmirror->pt, pfn);
 		page = xa_untag_pointer(entry);
 		if (!page)
 			return -ENOENT;
 
-		tmp = kmap(page);
-		memcpy(ptr, tmp, PAGE_SIZE);
-		kunmap(page);
+		memcpy_from_page(ptr, page, 0, PAGE_SIZE);
 
 		ptr += PAGE_SIZE;
 		bounce->cpages++;
@@ -437,16 +434,13 @@ static int dmirror_do_write(struct dmirror *dmirror, unsigned long start,
 	for (pfn = start >> PAGE_SHIFT; pfn < (end >> PAGE_SHIFT); pfn++) {
 		void *entry;
 		struct page *page;
-		void *tmp;
 
 		entry = xa_load(&dmirror->pt, pfn);
 		page = xa_untag_pointer(entry);
 		if (!page || xa_pointer_tag(entry) != DPT_XA_TAG_WRITE)
 			return -ENOENT;
 
-		tmp = kmap(page);
-		memcpy(tmp, ptr, PAGE_SIZE);
-		kunmap(page);
+		memcpy_to_page(page, 0, ptr, PAGE_SIZE);
 
 		ptr += PAGE_SIZE;
 		bounce->cpages++;
-- 
2.25.1