netdev - [PATCH net-next] net: msg_zerocopy: elide page accounting if RLIM

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Date:   Tue, 14 Feb 2023 10:57:40 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org, edumazet@...gle.com,
        pabeni@...hat.com, Willem de Bruijn <willemb@...gle.com>
Subject: [PATCH net-next] net: msg_zerocopy: elide page accounting if RLIM_INFINITY

From: Willem de Bruijn <willemb@...gle.com>

MSG_ZEROCOPY ensures that pinned user pages do not exceed the limit.
If no limit is set, skip this accounting as otherwise expensive
atomic_long operations are called for no reason.

This accounting is already skipped for privileged users. Rely on the
same mechanism: if no mmp->user is set, mm_unaccount_pinned_pages does
not decrement either.

Tested by running tools/testing/selftests/net/msg_zerocopy.sh with
an unprivileged user for the TXMODE binary:

    ip netns exec "${NS1}" sudo -u "{$USER}" "${BIN}" "-${IP}" ...

Signed-off-by: Willem de Bruijn <willemb@...gle.com>
---
 net/core/skbuff.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 13ea10cf8544..98ebce9f6a51 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1406,14 +1406,18 @@ EXPORT_SYMBOL_GPL(skb_morph);
 
 int mm_account_pinned_pages(struct mmpin *mmp, size_t size)
 {
-	unsigned long max_pg, num_pg, new_pg, old_pg;
+	unsigned long max_pg, num_pg, new_pg, old_pg, rlim;
 	struct user_struct *user;
 
 	if (capable(CAP_IPC_LOCK) || !size)
 		return 0;
 
+	rlim = rlimit(RLIMIT_MEMLOCK);
+	if (rlim == RLIM_INFINITY)
+		return 0;
+
 	num_pg = (size >> PAGE_SHIFT) + 2;	/* worst case */
-	max_pg = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
+	max_pg = rlim >> PAGE_SHIFT;
 	user = mmp->user ? : current_user();
 
 	old_pg = atomic_long_read(&user->locked_vm);
-- 
2.39.1.581.gbfd45094c4-goog