mm: Don't pin ZERO_PAGE in pin_user_pages()

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@redhat.com>
cc: Christoph Hellwig <hch@infradead.org>
cc: David Hildenbrand <david@redhat.com>
cc: Lorenzo Stoakes <lstoakes@gmail.com>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Matthew Wilcox <willy@infradead.org>
cc: Jan Kara <jack@suse.cz>
cc: Jeff Layton <jlayton@kernel.org>
cc: Jason Gunthorpe <jgg@nvidia.com>
cc: Logan Gunthorpe <logang@deltatee.com>
cc: Hillf Danton <hdanton@sina.com>
cc: Christian Brauner <brauner@kernel.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-kernel@vger.kernel.org
cc: linux-mm@kvack.org
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20230526214142.958751-2-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/Documentation/core-api/pin_user_pages.rst b/Documentation/core-api/pin_user_pages.rst
index 9fb0b1080d3b8cc7ae142fda425ebd01fe64ed0c..d3c1f6d8c0e0ec819843ad11bea22cad9614e657 100644 (file)
--- 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 27ce77080c79c7a026e641e491246fcf6f7e26c0..3c2f6b452586f233dfdbe1f0ce1580733430f2ee 100644 (file)
--- 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 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)