linux-kernel - [RFC PATCH v2] xen/privcmd: Convert get_user_pages*() to pin_user

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Date:   Tue, 23 Jun 2020 17:25:28 +0530
From:   Souptick Joarder <jrdr.linux@...il.com>
To:     jgross@...e.com, boris.ostrovsky@...cle.com, sstabellini@...nel.org
Cc:     xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        Souptick Joarder <jrdr.linux@...il.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: [RFC PATCH v2] xen/privcmd: Convert get_user_pages*() to pin_user_pages*()

In 2019, we introduced pin_user_pages*() and now we are converting
get_user_pages*() to the new API as appropriate. [1] & [2] could
be referred for more information. This is case 5 as per document [1].

As discussed, pages need to be marked as dirty before unpinned it.

Previously, if lock_pages() end up partially mapping pages, it used
to return -ERRNO due to which unlock_pages() have to go through
each pages[i] till *nr_pages* to validate them. This can be avoided
by passing correct number partially mapped pages & -ERRNO separately
while returning from lock_pages() due to error.
With this fix unlock_pages() doesn't need to validate pages[i] till
*nr_pages* for error scenario.

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

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

Signed-off-by: Souptick Joarder <jrdr.linux@...il.com>
Cc: John Hubbard <jhubbard@...dia.com>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
---
 drivers/xen/privcmd.c | 33 +++++++++++++++++++--------------
 1 file changed, 19 insertions(+), 14 deletions(-)

diff --git a/drivers/xen/privcmd.c b/drivers/xen/privcmd.c
index a250d11..eea90cd 100644
--- a/drivers/xen/privcmd.c
+++ b/drivers/xen/privcmd.c
@@ -580,25 +580,30 @@ static long privcmd_ioctl_mmap_batch(
 
 static int lock_pages(
 	struct privcmd_dm_op_buf kbufs[], unsigned int num,
-	struct page *pages[], unsigned int nr_pages)
+	struct page *pages[], unsigned int nr_pages, int *errno)
 {
 	unsigned int i;
+	int pinned = 0, rc = 0;
 
 	for (i = 0; i < num; i++) {
 		unsigned int requested;
-		int pinned;
 
+		rc += pinned;
 		requested = DIV_ROUND_UP(
 			offset_in_page(kbufs[i].uptr) + kbufs[i].size,
 			PAGE_SIZE);
-		if (requested > nr_pages)
-			return -ENOSPC;
+		if (requested > nr_pages) {
+			*errno = -ENOSPC;
+			return rc;
+		}
 
-		pinned = get_user_pages_fast(
+		pinned = pin_user_pages_fast(
 			(unsigned long) kbufs[i].uptr,
 			requested, FOLL_WRITE, pages);
-		if (pinned < 0)
-			return pinned;
+		if (pinned < 0) {
+			*errno = pinned;
+			return rc;
+		}
 
 		nr_pages -= pinned;
 		pages += pinned;
@@ -613,11 +618,7 @@ static void unlock_pages(struct page *pages[], unsigned int nr_pages)
 
 	if (!pages)
 		return;
-
-	for (i = 0; i < nr_pages; i++) {
-		if (pages[i])
-			put_page(pages[i]);
-	}
+	unpin_user_pages_dirty_lock(pages, nr_pages, 1);
 }
 
 static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
@@ -630,6 +631,7 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
 	struct xen_dm_op_buf *xbufs = NULL;
 	unsigned int i;
 	long rc;
+	int errno = 0;
 
 	if (copy_from_user(&kdata, udata, sizeof(kdata)))
 		return -EFAULT;
@@ -683,9 +685,12 @@ static long privcmd_ioctl_dm_op(struct file *file, void __user *udata)
 		goto out;
 	}
 
-	rc = lock_pages(kbufs, kdata.num, pages, nr_pages);
-	if (rc)
+	rc = lock_pages(kbufs, kdata.num, pages, nr_pages, &errno);
+	if (errno < 0) {
+		nr_pages = rc;
+		rc = errno;
 		goto out;
+	}
 
 	for (i = 0; i < kdata.num; i++) {
 		set_xen_guest_handle(xbufs[i].h, kbufs[i].uptr);
-- 
1.9.1