mm: Don't pin ZERO_PAGE in pin_user_pages()

[linux-block.git] / mm / gup.c

diff --git a/mm/gup.c b/mm/gup.c
index bbe4162365933e5d52f84c34037cb7d0949bc64d..ad28261dcafdcb4f373dd9b715047729eb7f5035 100644 (file)
--- 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)