netdev - [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user

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Message-ID: <20200601052633.853874-3-jhubbard@nvidia.com>
Date:   Sun, 31 May 2020 22:26:33 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Jonathan Corbet <corbet@....net>, <linux-doc@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, John Hubbard <jhubbard@...dia.com>
Subject: [PATCH v2 2/2] vhost: convert get_user_pages() --> pin_user_pages()

This code was using get_user_pages*(), in approximately a "Case 5"
scenario (accessing the data within a page), using the categorization
from [1]. That means that it's time to convert the get_user_pages*() +
put_page() calls to pin_user_pages*() + unpin_user_pages() calls.

There is some helpful background in [2]: basically, this is a small
part of fixing a long-standing disconnect between pinning pages, and
file systems' use of those pages.

[1] Documentation/core-api/pin_user_pages.rst

[2] "Explicit pinning of user-space pages":
    https://lwn.net/Articles/807108/

Cc: Michael S. Tsirkin <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: kvm@...r.kernel.org
Cc: virtualization@...ts.linux-foundation.org
Cc: netdev@...r.kernel.org
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 drivers/vhost/vhost.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 21a59b598ed8..596132a96cd5 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1762,15 +1762,14 @@ static int set_bit_to_user(int nr, void __user *addr)
 	int bit = nr + (log % PAGE_SIZE) * 8;
 	int r;
 
-	r = get_user_pages_fast(log, 1, FOLL_WRITE, &page);
+	r = pin_user_pages_fast(log, 1, FOLL_WRITE, &page);
 	if (r < 0)
 		return r;
 	BUG_ON(r != 1);
 	base = kmap_atomic(page);
 	set_bit(bit, base);
 	kunmap_atomic(base);
-	set_page_dirty_lock(page);
-	put_page(page);
+	unpin_user_pages_dirty_lock(&page, 1, true);
 	return 0;
 }
 
-- 
2.26.2