On 05.12.23 06:35, Vivek Kasireddy wrote:
For drivers that would like to longterm-pin the pages associated
with a memfd, 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 memfds but it should work with any files
that belong to either shmemfs 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
v5: (David)
- For hugetlb case, ensure that we only obtain head pages from the
mapping by using __filemap_get_folio() instead of find_get_page_flags()
- Handle -EEXIST when two or more potential users try to simultaneously
add a huge page to the mapping by forcing them to retry on failure
v6: (Christoph)
- Rename this API to memfd_pin_user_pages() to make it clear that it
is intended for memfds
- Move the memfd page allocation helper from gup.c to memfd.c
- Fix indentation errors in memfd_pin_user_pages()
- For contiguous ranges of folios, use a helper such as
filemap_get_folios_contig() to lookup the page cache in batches
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
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>
---
include/linux/memfd.h | 5 +++
include/linux/mm.h | 2 +
mm/gup.c | 102 ++++++++++++++++++++++++++++++++++++++++++
mm/memfd.c | 34 ++++++++++++++
4 files changed, 143 insertions(+)
diff --git a/include/linux/memfd.h b/include/linux/memfd.h
index e7abf6fa4c52..6fc0d1282151 100644
--- a/include/linux/memfd.h
+++ b/include/linux/memfd.h
@@ -6,11 +6,16 @@
#ifdef CONFIG_MEMFD_CREATE
extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int
arg);
+extern struct page *memfd_alloc_page(struct file *memfd, pgoff_t idx);
#else
static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned int a)
{
return -EINVAL;
}
+static inline struct page *memfd_alloc_page(struct file *memfd, pgoff_t
idx)
+{
+ return ERR_PTR(-EINVAL);
+}
#endif
#endif /* __LINUX_MEMFD_H */
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 418d26608ece..ac69db45509f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2472,6 +2472,8 @@ 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 memfd_pin_user_pages(struct file *file, pgoff_t start,
+ unsigned long nr_pages, 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 231711efa390..eb93d1ec9dc6 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -5,6 +5,7 @@
#include <linux/spinlock.h>
#include <linux/mm.h>
+#include <linux/memfd.h>
#include <linux/memremap.h>
#include <linux/pagemap.h>
#include <linux/rmap.h>
@@ -17,6 +18,7 @@
#include <linux/hugetlb.h>
#include <linux/migrate.h>
#include <linux/mm_inline.h>
+#include <linux/pagevec.h>
#include <linux/sched/mm.h>
#include <linux/shmem_fs.h>
@@ -3410,3 +3412,103 @@ long pin_user_pages_unlocked(unsigned long
start, unsigned long nr_pages,
&locked, gup_flags);
}
EXPORT_SYMBOL(pin_user_pages_unlocked);
+
+/**
+ * memfd_pin_user_pages() - pin user pages associated with a memfd
+ * @memfd: the memfd whose pages are to be pinned
+ * @start: starting memfd 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 memfd; given that a memfd is
either
+ * backed by shmem or hugetlb, the pages can either be found in the page
cache
+ * or need to be 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 memfd_pin_user_pages(struct file *memfd, pgoff_t start,
+ unsigned long nr_pages, struct page **pages)
+{
+ pgoff_t start_idx, end_idx = start + nr_pages - 1;
+ unsigned int flags, nr_folios, i, j;
+ struct folio_batch fbatch;
+ struct page *page = NULL;
+ struct folio *folio;
+ long ret;
+
+ if (!nr_pages)
+ return -EINVAL;
+
+ if (!memfd)
+ return -EINVAL;
+
+ if (!shmem_file(memfd) && !is_file_hugepages(memfd))
+ return -EINVAL;
+
+ flags = memalloc_pin_save();
+ do {
+ folio_batch_init(&fbatch);
+ start_idx = start;
+ i = 0;
+
+ while (start_idx <= end_idx) {
+ /*
+ * In most cases, we should be able to find the page
+ * in the page cache. If we cannot find it for some
+ * reason, we try to allocate one and add it to the
+ * page cache.
+ */
+ nr_folios = filemap_get_folios_contig(memfd-
f_mapping,
+ &start_idx,
+ end_idx,
+ &fbatch);
+ if (page) {
+ put_page(page);
+ page = NULL;
+ }
+ for (j = 0; j < nr_folios; j++) {
+ folio = fbatch.folios[j];
+ ret = try_grab_page(&folio->page, FOLL_PIN);
+ if (unlikely(ret)) {
+ folio_batch_release(&fbatch);
+ goto err;
+ }
+
+ pages[i++] = &folio->page;
+ }
I might be wrong, but that interface is still inconsistent. I think your
intention is to always return folios (head pages), but why are we
returning pages from this interface then?
It would be more consistent regarding the other GUP interfaces to return
the actual tail pages that fit the given "pgoff_t start". So if you
punch in "nr_pages" you expect to get "nr_pages" pages, and not some
other number of folios.
Otherwise, this interface is highly confusing.
If you always want to return folios, then better name it
"memfd_pin_user_folios" (or just "memfd_pin_folios") and pass in a range
(instead of a nr_pages parameter), and somehow indicate to the caller
how many folio were in that range, and if that range was fully covered.
