[PATCH v1 1/3] mm/gup: Introduce pin_user_pages_fd() for pinning shmem/hugetlbfs file pages

[Vivek Kasireddy]

For drivers that would like to longterm-pin the pages associated
with a file, the pin_user_pages_fd() API provides an option to
not only FOLL_PIN the pages but also to check and migrate them
if they reside in movable zone or CMA block. For now, this API
can only work with files belonging to shmem or hugetlbfs given
that the udmabuf driver is the only user.

It must be noted that the pages associated with hugetlbfs files
are expected to be found in the page cache. An error is returned
if they are not found. However, shmem pages can be swapped in or
allocated if they are not present in the page cache.

Cc: David Hildenbrand <david at redhat.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Mike Kravetz <mike.kravetz at oracle.com>
Cc: Hugh Dickins <hughd at google.com>
Cc: Peter Xu <peterx at redhat.com>
Cc: Gerd Hoffmann <kraxel at redhat.com>
Cc: Dongwon Kim <dongwon.kim at intel.com>
Cc: Junxiao Chang <junxiao.chang at intel.com>
Suggested-by: Jason Gunthorpe <jgg at nvidia.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy at intel.com>
---
 include/linux/mm.h |  2 ++
 mm/gup.c           | 87 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index bf5d0b1b16f4..af2121fb8101 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2457,6 +2457,8 @@ long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 		    struct page **pages, unsigned int gup_flags);
 long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 		    struct page **pages, unsigned int gup_flags);
+long pin_user_pages_fd(int fd, pgoff_t start, unsigned long nr_pages,
+		       unsigned int gup_flags, struct page **pages);
 
 int get_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages);
diff --git a/mm/gup.c b/mm/gup.c
index 2f8a2d89fde1..e34b77a15fa8 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -3400,3 +3400,90 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 				     &locked, gup_flags);
 }
 EXPORT_SYMBOL(pin_user_pages_unlocked);
+
+/**
+ * pin_user_pages_fd() - pin user pages associated with a file
+ * @fd:         the fd whose pages are to be pinned
+ * @start:      starting file offset
+ * @nr_pages:   number of pages from start to pin
+ * @gup_flags:  flags modifying pin behaviour
+ * @pages:      array that receives pointers to the pages pinned.
+ *              Should be at least nr_pages long.
+ *
+ * Attempt to pin (and migrate) pages associated with a file belonging to
+ * either shmem or hugetlbfs. An error is returned if pages associated with
+ * hugetlbfs files are not present in the page cache. However, shmem pages
+ * are swapped in or allocated if they are not present in the page cache.
+ *
+ * Returns number of pages pinned. This would be equal to the number of
+ * pages requested.
+ * If nr_pages is 0 or negative, returns 0. If no pages were pinned, returns
+ * -errno.
+ */
+long pin_user_pages_fd(int fd, pgoff_t start, unsigned long nr_pages,
+		       unsigned int gup_flags, struct page **pages)
+{
+	struct page *page;
+	struct file *filep;
+	unsigned int flags, i;
+	long ret;
+
+	if (nr_pages <= 0)
+		return 0;
+	if (!is_valid_gup_args(pages, NULL, &gup_flags, FOLL_PIN))
+		return 0;
+
+	if (start < 0)
+		return -EINVAL;
+
+	filep = fget(fd);
+	if (!filep)
+	    return -EINVAL;
+
+	if (!shmem_file(filep) && !is_file_hugepages(filep))
+	    return -EINVAL;
+
+	flags = memalloc_pin_save();
+	do {
+		for (i = 0; i < nr_pages; i++) {
+			if (shmem_mapping(filep->f_mapping)) {
+				page = shmem_read_mapping_page(filep->f_mapping,
+							       start + i);
+				if (IS_ERR(page)) {
+					ret = PTR_ERR(page);
+					goto err;
+				}
+			} else {
+				page = find_get_page_flags(filep->f_mapping,
+							   start + i,
+							   FGP_ACCESSED);
+				if (!page) {
+					ret = -EINVAL;
+					goto err;
+				}
+			}
+			ret = try_grab_page(page, FOLL_PIN);
+			if (unlikely(ret))
+				goto err;
+
+			pages[i] = page;
+			put_page(pages[i]);
+		}
+
+		ret = check_and_migrate_movable_pages(nr_pages, pages);
+	} while (ret == -EAGAIN);
+
+err:
+	memalloc_pin_restore(flags);
+	fput(filep);
+	if (!ret)
+		return nr_pages;
+
+	while (i > 0 && pages[--i]) {
+		unpin_user_page(pages[i]);
+		pages[i] = NULL;
+	}
+	return ret;
+}
+EXPORT_SYMBOL_GPL(pin_user_pages_fd);
+
-- 
2.39.2