linux-kernel - [PATCH] staging: lib: Use memcpy_to/from

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Message-ID: <20230608142553.GA341787@sumitra.com>
Date:   Thu, 8 Jun 2023 07:25:53 -0700
From:   Sumitra Sharma <sumitraartsy@...il.com>
To:     Jérôme Glisse <jglisse@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     Deepak R Varma <drv@...lo.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] staging: lib: Use memcpy_to/from_page()

Deprecate kmap() in favor of 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() offers thread-local and CPU-local mappings,
take pagefaults in a local kmap region and preserves preemption
by saving the mappings of outgoing task and restoring those of
the incoming one during a context switch.

It is faster than kmap(), and in functions "dmirror_do_read" and
"dmirror_do_write" the mappings are kept local to the thread.

Therefore, replace kmap/mem*()/kunmap pattern with
memcpy_to/from_page() and remove the unused variable "tmp".

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

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

 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..cc96dfa54122 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((char *)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, (char *)ptr, PAGE_SIZE);
 
 		ptr += PAGE_SIZE;
 		bounce->cpages++;
-- 
2.25.1