Re: [PATCH v4 1/3] mm: Don't pin ZERO_PAGE in pin_user_pages()

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 26.05.23 23:41, David Howells wrote:
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.

2 millions pins (FOLL_PIN, which increments the refcount by 1024) or 2 million references ?


Add a pair of functions to test whether a page or a folio is a ZERO_PAGE.


[...]

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))

Nit: Fits into a single line (without harming readability IMHO).

  			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.
   */

"it will not have pins in it" sounds fairly weird to a non-native speaker.

"Note that the refcount of any zero_pages returned among the pinned pages will not be incremented, and unpin_user_page() will similarly not decrement it."


Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Thanks,

David / dhildenb




[Index of Archives]     [Linux RAID]     [Linux SCSI]     [Linux ATA RAID]     [IDE]     [Linux Wireless]     [Linux Kernel]     [ATH6KL]     [Linux Bluetooth]     [Linux Netdev]     [Kernel Newbies]     [Security]     [Git]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Device Mapper]

  Powered by Linux