Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer
to it from the page tables and make unpin_user_page*() correspondingly
ignore a ZERO_PAGE when unpinning. We don't want to risk overrunning a
zero page's refcount as we're only allowed ~2 million pins on it -
something that userspace can conceivably trigger.
Add a pair of functions to test whether a page or a folio is a ZERO_PAGE.
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
cc: David Hildenbrand <david@xxxxxxxxxx>
cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
cc: Jens Axboe <axboe@xxxxxxxxx>
cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
cc: Jan Kara <jack@xxxxxxx>
cc: Jeff Layton <jlayton@xxxxxxxxxx>
cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
cc: Hillf Danton <hdanton@xxxxxxxx>
cc: Christian Brauner <brauner@xxxxxxxxxx>
cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
cc: linux-fsdevel@xxxxxxxxxxxxxxx
cc: linux-block@xxxxxxxxxxxxxxx
cc: linux-kernel@xxxxxxxxxxxxxxx
cc: linux-mm@xxxxxxxxx
---
Notes:
ver #3)
- Move is_zero_page() and is_zero_folio() to mm.h for dependency reasons.
- Add more comments and adjust the docs.
ver #2)
- Fix use of ZERO_PAGE().
- Add is_zero_page() and is_zero_folio() wrappers.
- Return the zero page obtained, not ZERO_PAGE(0) unconditionally.
Documentation/core-api/pin_user_pages.rst | 6 +++++
include/linux/mm.h | 26 +++++++++++++++++--
mm/gup.c | 31 ++++++++++++++++++++++-
3 files changed, 60 insertions(+), 3 deletions(-)
diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 9fb0b1080d3b..d3c1f6d8c0e0 100644
--- a/Documentation/core-api/pin_user_pages.rst
+++ b/Documentation/core-api/pin_user_pages.rst
@@ -112,6 +112,12 @@ pages:
This also leads to limitations: there are only 31-10==21 bits available for a
counter that increments 10 bits at a time.
+* Because of that limitation, special handling is applied to the zero pages
+ when using FOLL_PIN. We only pretend to pin a zero page - we don't alter its
+ refcount or pincount at all (it is permanent, so there's no need). The
+ unpinning functions also don't do anything to a zero page. This is
+ transparent to the caller.
+
* Callers must specifically request "dma-pinned tracking of pages". In other
words, just calling get_user_pages() will not suffice; a new set of functions,
pin_user_page() and related, must be used.
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 27ce77080c79..3c2f6b452586 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1910,6 +1910,28 @@ static inline bool page_needs_cow_for_dma(struct vm_area_struct *vma,
return page_maybe_dma_pinned(page);
}
+/**
+ * is_zero_page - Query if a page is a zero page
+ * @page: The page to query
+ *
+ * This returns true if @page is one of the permanent zero pages.
+ */
+static inline bool is_zero_page(const struct page *page)
+{
+ return is_zero_pfn(page_to_pfn(page));
+}
+
+/**
+ * is_zero_folio - Query if a folio is a zero page
+ * @folio: The folio to query
+ *
+ * This returns true if @folio is one of the permanent zero pages.
+ */
+static inline bool is_zero_folio(const struct folio *folio)
+{
+ return is_zero_page(&folio->page);
+}
+
/* MIGRATE_CMA and ZONE_MOVABLE do not allow pin pages */
#ifdef CONFIG_MIGRATION
static inline bool is_longterm_pinnable_page(struct page *page)
@@ -1920,8 +1942,8 @@ static inline bool is_longterm_pinnable_page(struct page *page)
if (mt == MIGRATE_CMA || mt == MIGRATE_ISOLATE)
return false;
#endif
- /* The zero page may always be pinned */
- if (is_zero_pfn(page_to_pfn(page)))
+ /* The zero page can be "pinned" but gets special handling. */
+ if (is_zero_page(page))
return true;
/* Coherent device memory must always allow eviction. */
diff --git a/mm/gup.c b/mm/gup.c
index bbe416236593..ad28261dcafd 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -51,7 +51,8 @@ static inline void sanity_check_pinned_pages(struct page **pages,
struct page *page = *pages;
struct folio *folio = page_folio(page);
- if (!folio_test_anon(folio))
+ if (is_zero_page(page) ||
+ !folio_test_anon(folio))
continue;
if (!folio_test_large(folio) || folio_test_hugetlb(folio))
VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
@@ -131,6 +132,13 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
else if (flags & FOLL_PIN) {
struct folio *folio;
+ /*
+ * Don't take a pin on the zero page - it's not going anywhere
+ * and it is used in a *lot* of places.
+ */
+ if (is_zero_page(page))
+ return page_folio(page);
+
/*
* Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
* right zone, so fail and let the caller fall back to the slow
@@ -180,6 +188,8 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
{
if (flags & FOLL_PIN) {
+ if (is_zero_folio(folio))
+ return;
node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
if (folio_test_large(folio))
atomic_sub(refs, &folio->_pincount);
@@ -224,6 +234,13 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
if (flags & FOLL_GET)
folio_ref_inc(folio);
else if (flags & FOLL_PIN) {
+ /*
+ * Don't take a pin on the zero page - it's not going anywhere
+ * and it is used in a *lot* of places.
+ */
+ if (is_zero_page(page))
+ return 0;
+
/*
* Similar to try_grab_folio(): be sure to *also*
* increment the normal page refcount field at least once,
@@ -3079,6 +3096,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
*
* FOLL_PIN means that the pages must be released via unpin_user_page(). Please
* see Documentation/core-api/pin_user_pages.rst for further details.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page() will not remove pins from it.
*/
int pin_user_pages_fast(unsigned long start, int nr_pages,
unsigned int gup_flags, struct page **pages)
@@ -3110,6 +3130,9 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast);
*
* FOLL_PIN means that the pages must be released via unpin_user_page(). Please
* see Documentation/core-api/pin_user_pages.rst for details.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page*() will not remove pins from it.
*/
long pin_user_pages_remote(struct mm_struct *mm,
unsigned long start, unsigned long nr_pages,
@@ -3143,6 +3166,9 @@ EXPORT_SYMBOL(pin_user_pages_remote);
*
* FOLL_PIN means that the pages must be released via unpin_user_page(). Please
* see Documentation/core-api/pin_user_pages.rst for details.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page*() will not remove pins from it.
*/
long pin_user_pages(unsigned long start, unsigned long nr_pages,
unsigned int gup_flags, struct page **pages,
@@ -3161,6 +3187,9 @@ EXPORT_SYMBOL(pin_user_pages);
* pin_user_pages_unlocked() is the FOLL_PIN variant of
* get_user_pages_unlocked(). Behavior is the same, except that this one sets
* FOLL_PIN and rejects FOLL_GET.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page*() will not remove pins from it.
*/
long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
struct page **pages, unsigned int gup_flags)
