On 18.11.23 07:32, Vivek Kasireddy wrote:
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 pin the pages via FOLL_PIN but also to check and migrate
them if they reside in movable zone or CMA block. This API
currently works with files that belong to either shmem or hugetlbfs.
Files belonging to other filesystems are rejected for now.
The pages need to be located first before pinning them via FOLL_PIN.
If they are found in the page cache, they can be immediately pinned.
Otherwise, they need to be allocated using the filesystem specific
APIs and then pinned.
v2:
- Drop gup_flags and improve comments and commit message (David)
- Allocate a page if we cannot find in page cache for the hugetlbfs
case as well (David)
- Don't unpin pages if there is a migration related failure (David)
- Drop the unnecessary nr_pages <= 0 check (Jason)
- Have the caller of the API pass in file * instead of fd (Jason)
v3: (David)
- Enclose the huge page allocation code with #ifdef CONFIG_HUGETLB_PAGE
(Build error reported by kernel test robot <lkp@xxxxxxxxx>)
- Don't forget memalloc_pin_restore() on non-migration related errors
- Improve the readability of the cleanup code associated with
non-migration related errors
- Augment the comments by describing FOLL_LONGTERM like behavior
- Include the R-b tag from Jason
v4:
- Remove the local variable "page" and instead use 3 return statements
in alloc_file_page() (David)
- Add the R-b tag from David
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx>
Cc: Dongwon Kim <dongwon.kim@xxxxxxxxx>
Cc: Junxiao Chang <junxiao.chang@xxxxxxxxx>
Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx> (v2)
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx> (v3)
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx>
---
[...]
+static struct page *alloc_file_page(struct file *file, pgoff_t idx)
+{
+#ifdef CONFIG_HUGETLB_PAGE
+ struct folio *folio;
+ int err;
+
+ if (is_file_hugepages(file)) {
+ folio = alloc_hugetlb_folio_nodemask(hstate_file(file),
+ NUMA_NO_NODE,
+ NULL,
+ GFP_USER);
+ if (folio && folio_try_get(folio)) {
+ err = hugetlb_add_to_page_cache(folio,
+ file->f_mapping,
+ idx);
+ if (err) {
+ folio_put(folio);
+ free_huge_folio(folio);
+ return ERR_PTR(err);
+ }
+ return &folio->page;
While looking at the user of pin_user_pages_fd(), I realized something:
Assume idx is not aligned to the hugetlb page size.
find_get_page_flags() would always return a tail page in that case, but
you'd be returning the head page here.
See pagecache_get_page()->folio_file_page(folio, index);
+ }
+ return ERR_PTR(-ENOMEM);
+ }
+#endif
+ return shmem_read_mapping_page(file->f_mapping, idx);
+}
+
+/**
+ * pin_user_pages_fd() - pin user pages associated with a file
+ * @file: the file whose pages are to be pinned
+ * @start: starting file offset
+ * @nr_pages: number of pages from start to pin
+ * @pages: array that receives pointers to the pages pinned.
+ * Should be at-least nr_pages long.
+ *
+ * Attempt to pin pages associated with a file that belongs to either shmem
+ * or hugetlb. The pages are either found in the page cache or allocated if
+ * necessary. Once the pages are located, they are all pinned via FOLL_PIN.
+ * And, these pinned pages need to be released either using unpin_user_pages()
+ * or unpin_user_page().
+ *
+ * It must be noted that the pages may be pinned for an indefinite amount
+ * of time. And, in most cases, the duration of time they may stay pinned
+ * would be controlled by the userspace. This behavior is effectively the
+ * same as using FOLL_LONGTERM with other GUP APIs.
+ *
+ * Returns number of pages pinned. This would be equal to the number of
+ * pages requested. If no pages were pinned, it returns -errno.
+ */
+long pin_user_pages_fd(struct file *file, pgoff_t start,
+ unsigned long nr_pages, struct page **pages)
+{
+ struct page *page;
+ unsigned int flags, i;
+ long ret;
+
+ if (start < 0)
+ return -EINVAL;
+
+ if (!file)
+ return -EINVAL;
+
+ if (!shmem_file(file) && !is_file_hugepages(file))
+ return -EINVAL;
+
+ flags = memalloc_pin_save();
+ do {
+ for (i = 0; i < nr_pages; i++) {
+ /*
+ * In most cases, we should be able to find the page
+ * in the page cache. If we cannot find it, we try to
+ * allocate one and add it to the page cache.
+ */
+ page = find_get_page_flags(file->f_mapping,
+ start + i,
+ FGP_ACCESSED);
+ if (!page) {
+ page = alloc_file_page(file, start + i);
+ if (IS_ERR(page)) {
+ ret = PTR_ERR(page);
+ goto err;
While looking at above, I do wonder: what if two parties tried to alloc
the page at the same time? I suspect we'd want to handle -EEXIST a bit
nicer here, right?
--
Cheers,
David / dhildenb
