linux-kernel - [PATCH v2] fs: Replace kmap{,_atomic}() with kmap_local

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Date:   Wed,  3 Aug 2022 20:28:56 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Ira Weiny <ira.weiny@...el.com>
Subject: [PATCH v2] fs: Replace kmap{,_atomic}() with kmap_local_page()

The use of kmap() and kmap_atomic() are being deprecated in favor of
kmap_local_page().

There are 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. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and are still valid.

Since the use of kmap_local_page() in exec.c is safe, it should be
preferred everywhere in exec.c.

As said, since kmap_local_page() can be also called from atomic context,
and since remove_arg_zero() doesn't (and shouldn't ever) rely on an
implicit preempt_disable(), this function can also safely replace
kmap_atomic().

Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in
fs/exec.c.

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

Cc: Eric W. Biederman <ebiederm@...ssion.com>
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>
---

v1->v2: Added more information to the commit log to address some
objections expressed by Eric W. Biederman[1] in reply to v1. No changes
have been made to the code. Forwarded a tag from Ira Weiny (thanks!).

[1] https://lore.kernel.org/lkml/8735fmqcfz.fsf@email.froward.int.ebiederm.org/

 fs/exec.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 5fd73915c62c..b51dd14e7388 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -584,11 +584,11 @@ static int copy_strings(int argc, struct user_arg_ptr argv,

 				if (kmapped_page) {
 					flush_dcache_page(kmapped_page);
-					kunmap(kmapped_page);
+					kunmap_local(kaddr);
 					put_arg_page(kmapped_page);
 				}
 				kmapped_page = page;
-				kaddr = kmap(kmapped_page);
+				kaddr = kmap_local_page(kmapped_page);
 				kpos = pos & PAGE_MASK;
 				flush_arg_page(bprm, kpos, kmapped_page);
 			}
@@ -602,7 +602,7 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
 out:
 	if (kmapped_page) {
 		flush_dcache_page(kmapped_page);
-		kunmap(kmapped_page);
+		kunmap_local(kaddr);
 		put_arg_page(kmapped_page);
 	}
 	return ret;
@@ -880,11 +880,11 @@ int transfer_args_to_stack(struct linux_binprm *bprm,

 	for (index = MAX_ARG_PAGES - 1; index >= stop; index--) {
 		unsigned int offset = index == stop ? bprm->p & ~PAGE_MASK : 0;
-		char *src = kmap(bprm->page[index]) + offset;
+		char *src = kmap_local_page(bprm->page[index]) + offset;
 		sp -= PAGE_SIZE - offset;
 		if (copy_to_user((void *) sp, src, PAGE_SIZE - offset) != 0)
 			ret = -EFAULT;
-		kunmap(bprm->page[index]);
+		kunmap_local(src);
 		if (ret)
 			goto out;
 	}
@@ -1683,13 +1683,13 @@ int remove_arg_zero(struct linux_binprm *bprm)
 			ret = -EFAULT;
 			goto out;
 		}
-		kaddr = kmap_atomic(page);
+		kaddr = kmap_local_page(page);

 		for (; offset < PAGE_SIZE && kaddr[offset];
 				offset++, bprm->p++)
 			;

-		kunmap_atomic(kaddr);
+		kunmap_local(kaddr);
 		put_arg_page(page);
 	} while (offset == PAGE_SIZE);

-- 
2.37.1