On 12.12.23 08:38, Vivek Kasireddy wrote:
For drivers that would like to longterm-pin the folios associated
with a memfd, the memfd_pin_folios() API provides an option to
not only pin the folios 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 folios 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
v7:
- Rename this API to memfd_pin_folios() and make it return folios
and offsets instead of pages (David)
- Don't continue processing the folios in the batch returned by
filemap_get_folios_contig() if they do not have correct next_idx
- Add the R-b tag from Christoph
Sorry, I'm still not happy about the current state, because (1) the
folio vs. pages handling is still mixed (2) we're returning+pinning a
large folio multiple times.
See below if there is an easy way to clean this up.
@@ -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 +3412,156 @@ long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
&locked, gup_flags);
}
EXPORT_SYMBOL(pin_user_pages_unlocked);
+
+/**
+ * memfd_pin_folios() - pin folios associated with a memfd
+ * @memfd: the memfd whose folios are to be pinned
+ * @start: starting memfd offset
+ * @nr_pages: number of pages from start to pin
We're not pinning pages. An inclusive range [start, end] would be clearer.
+ * @folios: array that receives pointers to the folios pinned.
+ * Should be at-least nr_pages long.
+ * @offsets: array that receives offsets of pages in their folios.
+ * Should be at-least nr_pages long.
See below, I'm wondering if this is really required once we return each folio
only once.
+ *
+ * Attempt to pin folios associated with a memfd; given that a memfd is
+ * either backed by shmem or hugetlb, the folios can either be found in
+ * the page cache or need to be allocated if necessary. Once the folios
+ * are located, they are all pinned via FOLL_PIN and the @offsets array
+ * is populated with offsets of the pages in their respective folios.
+ * Therefore, for each page the caller requested, there will be a
+ * corresponding entry in both @folios and @offsets. And, eventually,
+ * these pinned folios need to be released either using unpin_user_pages()
+ * or unpin_user_page().
Oh, we don't have a folio function yet? Should be easy to add, and we'd really
add them.
+ *
+ * It must be noted that the folios 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 folios pinned. This would be equal to the number of
+ * pages requested. If no folios were pinned, it returns -errno.
+ */
+long memfd_pin_folios(struct file *memfd, unsigned long start,
+ unsigned long nr_pages, struct folio **folios,
+ pgoff_t *offsets)
+{
+ unsigned long end = start + (nr_pages << PAGE_SHIFT) - 1;
+ unsigned int max_pgs, pgoff, pgshift = PAGE_SHIFT;
+ pgoff_t start_idx, end_idx, next_idx;
+ unsigned int flags, nr_folios, i, j;
+ struct folio *folio = NULL;
+ struct folio_batch fbatch;
+ struct page **pages;
+ struct hstate *h;
+ long ret;
+
+ if (!nr_pages)
+ return -EINVAL;
+
+ if (!memfd)
+ return -EINVAL;
+
+ if (!shmem_file(memfd) && !is_file_hugepages(memfd))
+ return -EINVAL;
+
+ pages = kmalloc_array(nr_pages, sizeof(*pages), GFP_KERNEL);
+ if (!pages)
+ return -ENOMEM;
+
+ if (is_file_hugepages(memfd)) {
+ h = hstate_file(memfd);
+ pgshift = huge_page_shift(h);
+ }
+
+ flags = memalloc_pin_save();
+ do {
+ i = 0;
+ start_idx = start >> pgshift;
+ end_idx = end >> pgshift;
+ if (is_file_hugepages(memfd)) {
+ start_idx <<= huge_page_order(h);
+ end_idx <<= huge_page_order(h);
+ }
+
+ folio_batch_init(&fbatch);
+ while (start_idx <= end_idx) {
+ /*
+ * In most cases, we should be able to find the folios
+ * in the page cache. If we cannot find them for some
+ * reason, we try to allocate them and add them to the
+ * page cache.
+ */
+ nr_folios = filemap_get_folios_contig(memfd->f_mapping,
+ &start_idx,
+ end_idx,
+ &fbatch);
+ if (folio) {
+ folio_put(folio);
+ folio = NULL;
+ }
+
+ next_idx = 0;
+ for (j = 0; j < nr_folios; j++) {
+ if (next_idx &&
+ next_idx != folio_index(fbatch.folios[j]))
+ continue;
+
+ folio = try_grab_folio(&fbatch.folios[j]->page,
+ 1, FOLL_PIN);
+ if (!folio) {
+ folio_batch_release(&fbatch);
+ kfree(pages);
+ goto err;
+ }
+
+ max_pgs = folio_nr_pages(folio);
+ if (i == 0) {
+ pgoff = offset_in_folio(folio, start);
+ pgoff >>= PAGE_SHIFT;
+ }
+
+ do {
+ folios[i] = folio;
+ offsets[i] = pgoff << PAGE_SHIFT;
+ pages[i] = folio_page(folio, 0);
+ folio_add_pin(folio);
+
+ pgoff++;
+ i++;
+ } while (pgoff < max_pgs && i < nr_pages);
+
+ pgoff = 0;
+ next_idx = folio_next_index(folio);
+ gup_put_folio(folio, 1, FOLL_PIN);
+ }
+
+ folio = NULL;
+ folio_batch_release(&fbatch);
+ if (!nr_folios) {
+ folio = memfd_alloc_folio(memfd, start_idx);
+ if (IS_ERR(folio)) {
+ ret = PTR_ERR(folio);
+ if (ret != -EEXIST) {
+ kfree(pages);
+ goto err;
+ }
+ }
+ }
+ }
+
+ ret = check_and_migrate_movable_pages(nr_pages, pages);
Having a folio variant would avoid having to mess with pages here at all.
Further, we're now returning+pinning the same folio multiple times, instead of
just once like the folio batching variant would.
I'm wondering if the following wouldn't make more sense, assuming we add
check_and_migrate_movable_folios(), which should be pretty easy to add.
Obviously untested, just to express what I have in mind:
/**
* memfd_pin_folios() - pin folios associated with a memfd
* @memfd: the memfd whose folios are to be pinned
* @start: the starting memfd offset
* @end: the final memfd offset (inclusive)
* @folios: array that receives pointers to the folios pinned
* @max_folios: the number of entries in the array for folios
* @offsets: the offset into the first folio
*
* Attempt to pin folios associated with a memfd; given that a memfd is
* either backed by shmem or hugetlb, the folios can either be found in
* the page cache or need to be allocated if necessary. Once the folios
* are located, they are all pinned via FOLL_PIN and @offset is populated
* with the offset into the first folio.
*
* Pinned folios must be released using unpin_folio() or unpin_folios().
*
* It must be noted that the folios 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 folios pinned, which might be less than @max_folios
* only if the whole range was pinned. If no folios were pinned, it returns
* -errno.
*/
long memfd_pin_folios(struct file *memfd, unsigned long start,
unsigned long end, struct folio **folios,
unsigned int max_folios, unsigned long *offset)
{
unsigned int pgshift = PAGE_SHIFT;
unsigned int flags, nr_folios, cur_folios, i;
pgoff_t start_idx, end_idx;
struct folio_batch fbatch;
struct folio *folio;
struct hstate *h;
long ret;
if (start > end || !max_folios)
return -EINVAL;
if (!memfd)
return -EINVAL;
if (!shmem_file(memfd) && !is_file_hugepages(memfd))
return -EINVAL;
if (is_file_hugepages(memfd)) {
h = hstate_file(memfd);
pgshift = huge_page_shift(h);
}
flags = memalloc_pin_save();
folio_batch_init(&fbatch);
do {
nr_folios = 0;
start_idx = start >> pgshift;
end_idx = end >> pgshift;
if (is_file_hugepages(memfd)) {
start_idx <<= huge_page_order(h);
end_idx <<= huge_page_order(h);
}
while (start_idx <= end_idx) {
/*
* In most cases, we should be able to find the folios
* in the page cache. If we cannot find them for some
* reason, we try to allocate them and add them to the
* page cache.
*/
folio_batch_release(&fbatch);
cur_folios = filemap_get_folios_contig(memfd->f_mapping,
&start_idx,
end_idx,
&fbatch);
if (!cur_folios) {
folio = memfd_alloc_folio(memfd, start_idx);
if (IS_ERR(folio)) {
ret = PTR_ERR(folio);
if (ret != -EEXIST)
goto err;
}
folio_put(folio);
continue;
}
/* Let's pin each folio, which shouldn't really fail. */
for (i = 0; i < cur_folios; i++) {
folio = try_grab_folio(&fbatch.folios[i]->page,
1, FOLL_PIN);
if (!folio)
goto err;
if (!nr_folios)
*offset = offset_in_folio(folio, start);
folios[nr_folios++] = folio;
if (max_folios == nr_folios)
break;
}
if (max_folios == nr_folios)
break;
}
folio_batch_release(&fbatch);
ret = check_and_migrate_movable_folios(nr_folios, folios);
} while (ret == -EAGAIN);
memalloc_pin_restore(flags);
return ret ? ret : nr_folios;
err:
folio_batch_release(&fbatch);
memalloc_pin_restore(flags);
while (i-- > 0)
if (folios[i])
gup_put_folio(folios[i], 1, FOLL_PIN);
return ret;
}
EXPORT_SYMBOL_GPL(memfd_pin_folios);
I'm still wondering about the offset handling, though. Could it happen that why we are
repeatedly calling filemap_get_folios_contig(), that we would need offset!=0 on any of
the other folios besides the first one? My current understanding (and looking at
filemap_get_folios_contig()) is: no.
I'm primarily concerned about concurrent fallocate(PUNCH_HOLE) and THP collapse/splitting.
--
Cheers,
David / dhildenb
