[PATCH v2 4/7] iov_iter: new iov_iter_pin_pages*() routines

John Hubbard
2022-08-31T04:19:14Z

Provide two new wrapper routines that are intended for user space pages
only:

iov_iter_pin_pages()
iov_iter_pin_pages_alloc()

Internally, these routines call pin_user_pages_fast(), instead of
get_user_pages_fast(), for user_backed_iter(i) and iov_iter_bvec(i)
cases.

As always, callers must use unpin_user_pages() or a suitable FOLL_PIN
variant, to release the pages, if they actually were acquired via
pin_user_pages_fast().

This is a prerequisite to converting bio/block layers over to use
pin_user_pages_fast().

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
include/linux/uio.h | 4 +++
lib/iov_iter.c | 86 +++++++++++++++++++++++++++++++++++++++++----
2 files changed, 84 insertions(+), 6 deletions(-)

diff --git a/include/linux/uio.h b/include/linux/uio.h
index 5896af36199c..e26908e443d1 100644
--- a/include/linux/uio.h
+++ b/include/linux/uio.h
@@ -251,6 +251,10 @@ ssize_t iov_iter_get_pages2(struct iov_iter *i, struct page **pages,
size_t maxsize, unsigned maxpages, size_t *start);
ssize_t iov_iter_get_pages_alloc2(struct iov_iter *i, struct page ***pages,
size_t maxsize, size_t *start);
+ssize_t iov_iter_pin_pages(struct iov_iter *i, struct page **pages,
+ size_t maxsize, unsigned int maxpages, size_t *start);
+ssize_t iov_iter_pin_pages_alloc(struct iov_iter *i, struct page ***pages,
+ size_t maxsize, size_t *start);
int iov_iter_npages(const struct iov_iter *i, int maxpages);
void iov_iter_restore(struct iov_iter *i, struct iov_iter_state *state);

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 4b7fce72e3e5..c63ce0eadfcb 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1425,9 +1425,31 @@ static struct page *first_bvec_segment(const struct iov_iter *i,
return page;
}

+enum pages_alloc_internal_flags {
+ USE_FOLL_GET,
+ MAYBE_USE_FOLL_PIN
+};
+
+/*
+ * Pins pages, either via get_page(), or via pin_user_page*(). The caller is
+ * responsible for tracking which pinning mechanism was used here, and releasing
+ * pages via the appropriate call: put_page() or unpin_user_page().
+ *
+ * The way to figure that out is:
+ *
+ * a) If how_to_pin == FOLL_GET, then this routine will always pin via
+ * get_page().
+ *
+ * b) If how_to_pin == MAYBE_USE_FOLL_PIN, then this routine will pin via
+ * pin_user_page*() for either user_backed_iter(i) cases, or
+ * iov_iter_is_bvec(i) cases. However, for the other cases (pipe,
+ * xarray), pages will be pinned via get_page().
+ */
static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
struct page ***pages, size_t maxsize,
- unsigned int maxpages, size_t *start)
+ unsigned int maxpages, size_t *start,
+ enum pages_alloc_internal_flags how_to_pin)
+
{
unsigned int n;

@@ -1454,7 +1476,12 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
n = want_pages_array(pages, maxsize, *start, maxpages);
if (!n)
return -ENOMEM;
- res = get_user_pages_fast(addr, n, gup_flags, *pages);
+
+ if (how_to_pin == MAYBE_USE_FOLL_PIN)
+ res = pin_user_pages_fast(addr, n, gup_flags, *pages);
+ else
+ res = get_user_pages_fast(addr, n, gup_flags, *pages);
+
if (unlikely(res <= 0))
return res;
maxsize = min_t(size_t, maxsize, res * PAGE_SIZE - *start);
@@ -1470,8 +1497,13 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
if (!n)
return -ENOMEM;
p = *pages;
- for (int k = 0; k < n; k++)
- get_page(p[k] = page + k);
+ for (int k = 0; k < n; k++) {
+ p[k] = page + k;
+ if (how_to_pin == MAYBE_USE_FOLL_PIN)
+ pin_user_page(p[k]);
+ else
+ get_page(p[k]);
+ }
maxsize = min_t(size_t, maxsize, n * PAGE_SIZE - *start);
i->count -= maxsize;
i->iov_offset += maxsize;
@@ -1497,10 +1529,29 @@ ssize_t iov_iter_get_pages2(struct iov_iter *i,
return 0;
BUG_ON(!pages);

- return __iov_iter_get_pages_alloc(i, &pages, maxsize, maxpages, start);
+ return __iov_iter_get_pages_alloc(i, &pages, maxsize, maxpages, start,
+ USE_FOLL_GET);
}
EXPORT_SYMBOL(iov_iter_get_pages2);

+/*
+ * A FOLL_PIN variant that calls pin_user_pages_fast() instead of
+ * get_user_pages_fast().
+ */
+ssize_t iov_iter_pin_pages(struct iov_iter *i,
+ struct page **pages, size_t maxsize, unsigned int maxpages,
+ size_t *start)
+{
+ if (!maxpages)
+ return 0;
+ if (WARN_ON_ONCE(!pages))
+ return -EINVAL;
+
+ return __iov_iter_get_pages_alloc(i, &pages, maxsize, maxpages, start,
+ MAYBE_USE_FOLL_PIN);
+}
+EXPORT_SYMBOL(iov_iter_pin_pages);
+
ssize_t iov_iter_get_pages_alloc2(struct iov_iter *i,
struct page ***pages, size_t maxsize,
size_t *start)
@@ -1509,7 +1560,8 @@ ssize_t iov_iter_get_pages_alloc2(struct iov_iter *i,

*pages = NULL;

- len = __iov_iter_get_pages_alloc(i, pages, maxsize, ~0U, start);
+ len = __iov_iter_get_pages_alloc(i, pages, maxsize, ~0U, start,
+ USE_FOLL_GET);
if (len <= 0) {
kvfree(*pages);
*pages = NULL;
@@ -1518,6 +1570,28 @@ ssize_t iov_iter_get_pages_alloc2(struct iov_iter *i,
}
EXPORT_SYMBOL(iov_iter_get_pages_alloc2);

+/*
+ * A FOLL_PIN variant that calls pin_user_pages_fast() instead of
+ * get_user_pages_fast().
+ */
+ssize_t iov_iter_pin_pages_alloc(struct iov_iter *i,
+ struct page ***pages, size_t maxsize,
+ size_t *start)
+{
+ ssize_t len;
+
+ *pages = NULL;
+
+ len = __iov_iter_get_pages_alloc(i, pages, maxsize, ~0U, start,
+ MAYBE_USE_FOLL_PIN);
+ if (len <= 0) {
+ kvfree(*pages);
+ *pages = NULL;
+ }
+ return len;
+}
+EXPORT_SYMBOL(iov_iter_pin_pages_alloc);
+
size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum,
struct iov_iter *i)
{
--
2.37.2