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

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Message-Id: <20220718002645.28817-1-fmdefrancesco@gmail.com>
Date:   Mon, 18 Jul 2022 02:26:45 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Luis Chamberlain <mcgrof@...nel.org>,
        linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH] module: Replace kmap() with kmap_local_page()

kmap() is being deprecated in favor of kmap_local_page().

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. Its use in
module_gzip_decompress() and in module_xz_decompress() is safe and
should be preferred.

Therefore, replace kmap() with kmap_local_page().

Tested on a QEMU/KVM x86_32 VM with 4GB RAM, booting kernels with
HIGHMEM64GB enabled. Modules compressed with XZ or GZIP decompress
properly.

Suggested-by: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 kernel/module/decompress.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/module/decompress.c b/kernel/module/decompress.c
index 2fc7081dd7c1..4d0bcb3d9e44 100644
--- a/kernel/module/decompress.c
+++ b/kernel/module/decompress.c
@@ -119,10 +119,10 @@ static ssize_t module_gzip_decompress(struct load_info *info,
 			goto out_inflate_end;
 		}

-		s.next_out = kmap(page);
+		s.next_out = kmap_local_page(page);
 		s.avail_out = PAGE_SIZE;
 		rc = zlib_inflate(&s, 0);
-		kunmap(page);
+		kunmap_local(s.next_out);

 		new_size += PAGE_SIZE - s.avail_out;
 	} while (rc == Z_OK);
@@ -178,11 +178,11 @@ static ssize_t module_xz_decompress(struct load_info *info,
 			goto out;
 		}

-		xz_buf.out = kmap(page);
+		xz_buf.out = kmap_local_page(page);
 		xz_buf.out_pos = 0;
 		xz_buf.out_size = PAGE_SIZE;
 		xz_ret = xz_dec_run(xz_dec, &xz_buf);
-		kunmap(page);
+		kunmap_local(xz_buf.out);

 		new_size += xz_buf.out_pos;
 	} while (xz_buf.out_pos == PAGE_SIZE && xz_ret == XZ_OK);
-- 
2.37.1