linux-kernel - [PATCH v2 2/2] mm/gup: frame_vector: convert get_user_pages() --> pin_user

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Date:   Sun, 31 May 2020 16:41:31 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     David Hildenbrand <david@...hat.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        John Hubbard <jhubbard@...dia.com>
Subject: [PATCH v2 2/2] mm/gup: frame_vector: convert get_user_pages() --> pin_user_pages()

This code was using get_user_pages*(), and all of the callers so far
were in a "Case 2" scenario (DMA/RDMA), 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: David Hildenbrand <david@...hat.com>
Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 mm/frame_vector.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/frame_vector.c b/mm/frame_vector.c
index c431ca81dad5..4107dbca0056 100644
--- a/mm/frame_vector.c
+++ b/mm/frame_vector.c
@@ -72,7 +72,7 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames,
 	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP))) {
 		vec->got_ref = true;
 		vec->is_pfns = false;
-		ret = get_user_pages_locked(start, nr_frames,
+		ret = pin_user_pages_locked(start, nr_frames,
 			gup_flags, (struct page **)(vec->ptrs), &locked);
 		goto out;
 	}
@@ -122,7 +122,6 @@ EXPORT_SYMBOL(get_vaddr_frames);
  */
 void put_vaddr_frames(struct frame_vector *vec)
 {
-	int i;
 	struct page **pages;
 
 	if (!vec->got_ref)
@@ -135,8 +134,8 @@ void put_vaddr_frames(struct frame_vector *vec)
 	 */
 	if (WARN_ON(IS_ERR(pages)))
 		goto out;
-	for (i = 0; i < vec->nr_frames; i++)
-		put_page(pages[i]);
+
+	unpin_user_pages(pages, vec->nr_frames);
 	vec->got_ref = false;
 out:
 	vec->nr_frames = 0;
-- 
2.26.2