linux-kernel - [PATCH v2 3/3] fs/ecryptfs: Use kmap_local_page() in copy_up_encrypted_with

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Message-Id: <20230426172223.8896-4-fmdefrancesco@gmail.com>
Date:   Wed, 26 Apr 2023 19:22:23 +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 3/3] fs/ecryptfs: Use kmap_local_page() in copy_up_encrypted_with_header()

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

Therefore, replace kmap_atomic() with kmap_local_page() in
ecryptfs_copy_up_encrypted_with_header().

kmap_atomic() is implemented like a kmap_local_page() which also
disables page-faults and preemption (the latter only in !PREEMPT_RT
kernels). The kernel virtual addresses returned by these two API are
only valid in the context of the callers (i.e., they cannot be handed to
other threads).

With kmap_local_page() the mappings are per thread and CPU local like
in kmap_atomic(); however, they can handle page-faults and can be called
from any context (including interrupts). The tasks that call
kmap_local_page() can be preempted and, when they are scheduled to run
again, the kernel virtual addresses are restored and are still valid.

In ecryptfs_copy_up_encrypted_with_header(), the block of code between
the mapping and un-mapping does not depend on the above-mentioned side
effects of kmap_aatomic(), so that the mere replacements of the old API
with the new one is all that is required (i.e., there is no need to
explicitly call pagefault_disable() and/or preempt_disable()).

Tested in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
HIGHMEM64GB enabled.

Cc: Ira Weiny <ira.weiny@...el.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
---
 fs/ecryptfs/mmap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ecryptfs/mmap.c b/fs/ecryptfs/mmap.c
index 373c3e5747e6..cb1e998ce54d 100644
--- a/fs/ecryptfs/mmap.c
+++ b/fs/ecryptfs/mmap.c
@@ -125,7 +125,7 @@ ecryptfs_copy_up_encrypted_with_header(struct page *page,
 			/* This is a header extent */
 			char *page_virt;

-			page_virt = kmap_atomic(page);
+			page_virt = kmap_local_page(page);
 			memset(page_virt, 0, PAGE_SIZE);
 			/* TODO: Support more than one header extent */
 			if (view_extent_num == 0) {
@@ -138,7 +138,7 @@ ecryptfs_copy_up_encrypted_with_header(struct page *page,
 							       crypt_stat,
 							       &written);
 			}
-			kunmap_atomic(page_virt);
+			kunmap_local(page_virt);
 			flush_dcache_page(page);
 			if (rc) {
 				printk(KERN_ERR "%s: Error reading xattr "
-- 
2.40.0