linux-kernel - [PATCH v2 1/3] fs/ecryptfs: Replace kmap() with kmap_local

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Date:   Wed, 26 Apr 2023 19:22:21 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Tyler Hicks <code@...icks.com>,
        Christian Brauner <brauner@...nel.org>,
        Dave Chinner <dchinner@...hat.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Al Viro <viro@...iv.linux.org.uk>, ecryptfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH v2 1/3] fs/ecryptfs: Replace kmap() with kmap_local_page()

kmap() has been deprecated in favor of kmap_local_page().

Therefore, replace kmap() with kmap_local_page() in fs/ecryptfs.

There are two main problems with kmap(): (1) It comes with an overhead as
the 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. The tasks can
be preempted and, when they are scheduled to run again, the kernel
virtual addresses are restored and still valid.

Obviously, thread locality implies that the kernel virtual addresses
returned by kmap_local_page() are only valid in the context of the
callers (i.e., they cannot be handed to other threads).

The use of kmap_local_page() in fs/ecryptfs does not break the
above-mentioned assumption, so it is allowed and preferred.

Tested in 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>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 fs/ecryptfs/crypto.c     | 8 ++++----
 fs/ecryptfs/read_write.c | 8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/ecryptfs/crypto.c b/fs/ecryptfs/crypto.c
index c16f0d660cb7..03bd55069d86 100644
--- a/fs/ecryptfs/crypto.c
+++ b/fs/ecryptfs/crypto.c
@@ -441,10 +441,10 @@ int ecryptfs_encrypt_page(struct page *page)
 	}
 
 	lower_offset = lower_offset_for_page(crypt_stat, page);
-	enc_extent_virt = kmap(enc_extent_page);
+	enc_extent_virt = kmap_local_page(enc_extent_page);
 	rc = ecryptfs_write_lower(ecryptfs_inode, enc_extent_virt, lower_offset,
 				  PAGE_SIZE);
-	kunmap(enc_extent_page);
+	kunmap_local(enc_extent_virt);
 	if (rc < 0) {
 		ecryptfs_printk(KERN_ERR,
 			"Error attempting to write lower page; rc = [%d]\n",
@@ -490,10 +490,10 @@ int ecryptfs_decrypt_page(struct page *page)
 	BUG_ON(!(crypt_stat->flags & ECRYPTFS_ENCRYPTED));
 
 	lower_offset = lower_offset_for_page(crypt_stat, page);
-	page_virt = kmap(page);
+	page_virt = kmap_local_page(page);
 	rc = ecryptfs_read_lower(page_virt, lower_offset, PAGE_SIZE,
 				 ecryptfs_inode);
-	kunmap(page);
+	kunmap_local(page_virt);
 	if (rc < 0) {
 		ecryptfs_printk(KERN_ERR,
 			"Error attempting to read lower page; rc = [%d]\n",
diff --git a/fs/ecryptfs/read_write.c b/fs/ecryptfs/read_write.c
index 60bdcaddcbe5..5edf027c8359 100644
--- a/fs/ecryptfs/read_write.c
+++ b/fs/ecryptfs/read_write.c
@@ -64,11 +64,11 @@ int ecryptfs_write_lower_page_segment(struct inode *ecryptfs_inode,
 
 	offset = ((((loff_t)page_for_lower->index) << PAGE_SHIFT)
 		  + offset_in_page);
-	virt = kmap(page_for_lower);
+	virt = kmap_local_page(page_for_lower);
 	rc = ecryptfs_write_lower(ecryptfs_inode, virt, offset, size);
 	if (rc > 0)
 		rc = 0;
-	kunmap(page_for_lower);
+	kunmap_local(virt);
 	return rc;
 }
 
@@ -253,11 +253,11 @@ int ecryptfs_read_lower_page_segment(struct page *page_for_ecryptfs,
 	int rc;
 
 	offset = ((((loff_t)page_index) << PAGE_SHIFT) + offset_in_page);
-	virt = kmap(page_for_ecryptfs);
+	virt = kmap_local_page(page_for_ecryptfs);
 	rc = ecryptfs_read_lower(virt, offset, size, ecryptfs_inode);
 	if (rc > 0)
 		rc = 0;
-	kunmap(page_for_ecryptfs);
+	kunmap_local(virt);
 	flush_dcache_page(page_for_ecryptfs);
 	return rc;
 }
-- 
2.40.0