linux-kernel - [PATCH] evm: stop avoidably reading i_writecount in evm_file

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Message-ID: <20240806133607.869394-1-mjguzik@gmail.com>
Date: Tue,  6 Aug 2024 15:36:07 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: zohar@...ux.ibm.com,
	roberto.sassu@...wei.com,
	paul@...l-moore.com,
	jmorris@...ei.org,
	serge@...lyn.com
Cc: linux-kernel@...r.kernel.org,
	linux-integrity@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] evm: stop avoidably reading i_writecount in evm_file_release

The EVM_NEW_FILE flag is unset if the file already existed at the time
of open and this can be checked without looking at i_writecount.

Not accessing it reduces traffic on the cacheline during parallel open
of the same file and drop the evm_file_release routine from second place
to bottom of the profile.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

The context is that I'm writing a patch which removes one lockref
get/put cycle on parallel open. An operational WIP reduces ping-pong in
that area and made do_dentry_open skyrocket along with evm_file_release,
due to i_writecount access. With the patch they go down again and
apparmor takes the rightful first place.

The patch accounts for about 5% speed up at 20 cores running open3 from
will-it-scale on top of the above wip. (the apparmor + lockref thing
really don't scale, that's next)

I would provide better measurements, but the wip is not ready (as the
description suggests) and I need evm out of the way for the actual
patch.

 security/integrity/evm/evm_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/security/integrity/evm/evm_main.c b/security/integrity/evm/evm_main.c
index 62fe66dd53ce..309630f319e2 100644
--- a/security/integrity/evm/evm_main.c
+++ b/security/integrity/evm/evm_main.c
@@ -1084,7 +1084,8 @@ static void evm_file_release(struct file *file)
 	if (!S_ISREG(inode->i_mode) || !(mode & FMODE_WRITE))
 		return;

-	if (iint && atomic_read(&inode->i_writecount) == 1)
+	if (iint && iint->flags & EVM_NEW_FILE &&
+	    atomic_read(&inode->i_writecount) == 1)
 		iint->flags &= ~EVM_NEW_FILE;
 }

-- 
2.43.0