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Date:   Sat, 15 Aug 2020 20:42:52 +0800
Cc:,,, Coly Li <>,
        Chaitanya Kulkarni <>,
        Christoph Hellwig <>, Hannes Reinecke <>,
        Jan Kara <>, Jens Axboe <>,
        Mikhail Skorzhinskii <>,
        Philipp Reisner <>,
        Sagi Grimberg <>,
        Vlastimil Babka <>
Subject: [PATCH v4 1/3] net: introduce helper sendpage_ok() in include/linux/net.h

From: Coly Li <>

The original problem was from nvme-over-tcp code, who mistakenly uses
kernel_sendpage() to send pages allocated by __get_free_pages() without
__GFP_COMP flag. Such pages don't have refcount (page_count is 0) on
tail pages, sending them by kernel_sendpage() may trigger a kernel panic
from a corrupted kernel heap, because these pages are incorrectly freed
in network stack as page_count 0 pages.

This patch introduces a helper sendpage_ok(), it returns true if the
checking page,
- is not slab page: PageSlab(page) is false.
- has page refcount: page_count(page) is not zero

All drivers who want to send page to remote end by kernel_sendpage()
may use this helper to check whether the page is OK. If the helper does
not return true, the driver should try other non sendpage method (e.g.
sock_no_sendpage()) to handle the page.

Signed-off-by: Coly Li <>
Cc: Chaitanya Kulkarni <>
Cc: Christoph Hellwig <>
Cc: Hannes Reinecke <>
Cc: Jan Kara <>
Cc: Jens Axboe <>
Cc: Mikhail Skorzhinskii <>
Cc: Philipp Reisner <>
Cc: Sagi Grimberg <>
Cc: Vlastimil Babka <>
v4, change sendpage_ok() as an inline helper, and post it as
    separate patch.
v3, introduce a more common sendpage_ok()
v2, fix typo in patch subject
v1, the initial version.
 include/linux/net.h | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/include/linux/net.h b/include/linux/net.h
index d48ff1180879..97e8f1a8a427 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -286,6 +286,21 @@ do {									\
 #define net_get_random_once_wait(buf, nbytes)			\
 	get_random_once_wait((buf), (nbytes))
+ * E.g. XFS meta- & log-data is in slab pages, or bcache meta
+ * data pages, or other high order pages allocated by
+ * __get_free_pages() without __GFP_COMP, which have a page_count
+ * of 0 and/or have PageSlab() set. We cannot use send_page for
+ * those, as that does get_page(); put_page(); and would cause
+ * either a VM_BUG directly, or __page_cache_release a page that
+ * would actually still be referenced by someone, leading to some
+ * obscure delayed Oops somewhere else.
+ */
+static inline bool sendpage_ok(struct page *page)
+	return  (!PageSlab(page) && page_count(page) >= 1);
 int kernel_sendmsg(struct socket *sock, struct msghdr *msg, struct kvec *vec,
 		   size_t num, size_t len);
 int kernel_sendmsg_locked(struct sock *sk, struct msghdr *msg,

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