mm/gup: track FOLL_PIN pages

Add tracking of pages that were pinned via FOLL_PIN.  This tracking is
implemented via overloading of page->_refcount: pins are added by adding
GUP_PIN_COUNTING_BIAS (1024) to the refcount.  This provides a fuzzy
indication of pinning, and it can have false positives (and that's OK).
Please see the pre-existing Documentation/core-api/pin_user_pages.rst for
details.

As mentioned in pin_user_pages.rst, callers who effectively set FOLL_PIN
(typically via pin_user_pages*()) are required to ultimately free such
pages via unpin_user_page().

Please also note the limitation, discussed in pin_user_pages.rst under the
"TODO: for 1GB and larger huge pages" section.  (That limitation will be
removed in a following patch.)

The effect of a FOLL_PIN flag is similar to that of FOLL_GET, and may be
thought of as "FOLL_GET for DIO and/or RDMA use".

Pages that have been pinned via FOLL_PIN are identifiable via a new
function call:

   bool page_maybe_dma_pinned(struct page *page);

What to do in response to encountering such a page, is left to later
patchsets. There is discussion about this in [1], [2], [3], and [4].

This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().

[1] Some slow progress on get_user_pages() (Apr 2, 2019):
    https://lwn.net/Articles/784574/
[2] DMA and get_user_pages() (LPC: Dec 12, 2018):
    https://lwn.net/Articles/774411/
[3] The trouble with get_user_pages() (Apr 30, 2018):
    https://lwn.net/Articles/753027/
[4] LWN kernel index: get_user_pages():
    https://lwn.net/Kernel/Index/#Memory_management-get_user_pages

[jhubbard@nvidia.com: add kerneldoc]
  Link: http://lkml.kernel.org/r/20200307021157.235726-1-jhubbard@nvidia.com
[imbrenda@linux.ibm.com: if pin fails, we need to unpin, a simple put_page will not be enough]
  Link: http://lkml.kernel.org/r/20200306132537.783769-2-imbrenda@linux.ibm.com
[akpm@linux-foundation.org: fix put_compound_head defined but not used]
Suggested-by: Jan Kara <jack@suse.cz>
Suggested-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Link: http://lkml.kernel.org/r/20200211001536.1027652-7-jhubbard@nvidia.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/core-api/pin_user_pages.rst

@@ -173,8 +173,8 @@ CASE 4: Pinning for struct page manipulation only
 -------------------------------------------------
 Here, normal GUP calls are sufficient, so neither flag needs to be set.
 
-page_dma_pinned(): the whole point of pinning
-=============================================
+page_maybe_dma_pinned(): the whole point of pinning
+===================================================
 
 The whole point of marking pages as "DMA-pinned" or "gup-pinned" is to be able
 to query, "is this page DMA-pinned?" That allows code such as page_mkclean()
@@ -186,7 +186,7 @@ and debates (see the References at the end of this document). It's a TODO item
 here: fill in the details once that's worked out. Meanwhile, it's safe to say
 that having this available: ::
 
-        static inline bool page_dma_pinned(struct page *page)
+        static inline bool page_maybe_dma_pinned(struct page *page)
 
 ...is a prerequisite to solving the long-running gup+DMA problem.
 

include/linux/mm.h

@@ -1001,6 +1001,8 @@ static inline void get_page(struct page *page)
 	page_ref_inc(page);
 }
 
+bool __must_check try_grab_page(struct page *page, unsigned int flags);
+
 static inline __must_check bool try_get_page(struct page *page)
 {
 	page = compound_head(page);
@@ -1029,29 +1031,79 @@ static inline void put_page(struct page *page)
 		__put_page(page);
 }
 
-/**
- * unpin_user_page() - release a gup-pinned page
- * @page:            pointer to page to be released
+/*
+ * GUP_PIN_COUNTING_BIAS, and the associated functions that use it, overload
+ * the page's refcount so that two separate items are tracked: the original page
+ * reference count, and also a new count of how many pin_user_pages() calls were
+ * made against the page. ("gup-pinned" is another term for the latter).
+ *
+ * With this scheme, pin_user_pages() becomes special: such pages are marked as
+ * distinct from normal pages. As such, the unpin_user_page() call (and its
+ * variants) must be used in order to release gup-pinned pages.
+ *
+ * Choice of value:
+ *
+ * By making GUP_PIN_COUNTING_BIAS a power of two, debugging of page reference
+ * counts with respect to pin_user_pages() and unpin_user_page() becomes
+ * simpler, due to the fact that adding an even power of two to the page
+ * refcount has the effect of using only the upper N bits, for the code that
+ * counts up using the bias value. This means that the lower bits are left for
+ * the exclusive use of the original code that increments and decrements by one
+ * (or at least, by much smaller values than the bias value).
  *
- * Pages that were pinned via pin_user_pages*() must be released via either
- * unpin_user_page(), or one of the unpin_user_pages*() routines. This is so
- * that eventually such pages can be separately tracked and uniquely handled. In
- * particular, interactions with RDMA and filesystems need special handling.
+ * Of course, once the lower bits overflow into the upper bits (and this is
+ * OK, because subtraction recovers the original values), then visual inspection
+ * no longer suffices to directly view the separate counts. However, for normal
+ * applications that don't have huge page reference counts, this won't be an
+ * issue.
  *
- * unpin_user_page() and put_page() are not interchangeable, despite this early
- * implementation that makes them look the same. unpin_user_page() calls must
- * be perfectly matched up with pin*() calls.
+ * Locking: the lockless algorithm described in page_cache_get_speculative()
+ * and page_cache_gup_pin_speculative() provides safe operation for
+ * get_user_pages and page_mkclean and other calls that race to set up page
+ * table entries.
  */
-static inline void unpin_user_page(struct page *page)
-{
-	put_page(page);
-}
+#define GUP_PIN_COUNTING_BIAS (1U << 10)
 
+void unpin_user_page(struct page *page);
 void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
 				 bool make_dirty);
-
 void unpin_user_pages(struct page **pages, unsigned long npages);
 
+/**
+ * page_maybe_dma_pinned() - report if a page is pinned for DMA.
+ *
+ * This function checks if a page has been pinned via a call to
+ * pin_user_pages*().
+ *
+ * For non-huge pages, the return value is partially fuzzy: false is not fuzzy,
+ * because it means "definitely not pinned for DMA", but true means "probably
+ * pinned for DMA, but possibly a false positive due to having at least
+ * GUP_PIN_COUNTING_BIAS worth of normal page references".
+ *
+ * False positives are OK, because: a) it's unlikely for a page to get that many
+ * refcounts, and b) all the callers of this routine are expected to be able to
+ * deal gracefully with a false positive.
+ *
+ * For more information, please see Documentation/vm/pin_user_pages.rst.
+ *
+ * @page:	pointer to page to be queried.
+ * @Return:	True, if it is likely that the page has been "dma-pinned".
+ *		False, if the page is definitely not dma-pinned.
+ */
+static inline bool page_maybe_dma_pinned(struct page *page)
+{
+	/*
+	 * page_ref_count() is signed. If that refcount overflows, then
+	 * page_ref_count() returns a negative value, and callers will avoid
+	 * further incrementing the refcount.
+	 *
+	 * Here, for that overflow case, use the signed bit to count a little
+	 * bit higher via unsigned math, and thus still get an accurate result.
+	 */
+	return ((unsigned int)page_ref_count(compound_head(page))) >=
+		GUP_PIN_COUNTING_BIAS;
+}
+
 #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
 #define SECTION_IN_PAGE_FLAGS
 #endif

mm/huge_memory.c

@@ -958,6 +958,11 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 	 */
 	WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
 
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return NULL;
+
 	if (flags & FOLL_WRITE && !pmd_write(*pmd))
 		return NULL;
 
@@ -973,7 +978,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 	 * device mapped pages can only be returned if the
 	 * caller will manage the page reference count.
 	 */
-	if (!(flags & FOLL_GET))
+	if (!(flags & (FOLL_GET | FOLL_PIN)))
 		return ERR_PTR(-EEXIST);
 
 	pfn += (addr & ~PMD_MASK) >> PAGE_SHIFT;
@@ -981,7 +986,8 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 	if (!*pgmap)
 		return ERR_PTR(-EFAULT);
 	page = pfn_to_page(pfn);
-	get_page(page);
+	if (!try_grab_page(page, flags))
+		page = ERR_PTR(-ENOMEM);
 
 	return page;
 }
@@ -1101,6 +1107,11 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 	if (flags & FOLL_WRITE && !pud_write(*pud))
 		return NULL;
 
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return NULL;
+
 	if (pud_present(*pud) && pud_devmap(*pud))
 		/* pass */;
 	else
@@ -1112,8 +1123,10 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 	/*
 	 * device mapped pages can only be returned if the
 	 * caller will manage the page reference count.
+	 *
+	 * At least one of FOLL_GET | FOLL_PIN must be set, so assert that here:
 	 */
-	if (!(flags & FOLL_GET))
+	if (!(flags & (FOLL_GET | FOLL_PIN)))
 		return ERR_PTR(-EEXIST);
 
 	pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT;
@@ -1121,7 +1134,8 @@ struct page *follow_devmap_pud(struct vm_area_struct *vma, unsigned long addr,
 	if (!*pgmap)
 		return ERR_PTR(-EFAULT);
 	page = pfn_to_page(pfn);
-	get_page(page);
+	if (!try_grab_page(page, flags))
+		page = ERR_PTR(-ENOMEM);
 
 	return page;
 }
@@ -1497,8 +1511,13 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	page = pmd_page(*pmd);
 	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+
+	if (!try_grab_page(page, flags))
+		return ERR_PTR(-ENOMEM);
+
 	if (flags & FOLL_TOUCH)
 		touch_pmd(vma, addr, pmd, flags);
+
 	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
 		/*
 		 * We don't mlock() pte-mapped THPs. This way we can avoid
@@ -1535,8 +1554,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 skip_mlock:
 	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
 	VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
-	if (flags & FOLL_GET)
-		get_page(page);
 
 out:
 	return page;

mm/hugetlb.c

@@ -4375,19 +4375,6 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 		pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
 		page = pte_page(huge_ptep_get(pte));
 
-		/*
-		 * Instead of doing 'try_get_page()' below in the same_page
-		 * loop, just check the count once here.
-		 */
-		if (unlikely(page_count(page) <= 0)) {
-			if (pages) {
-				spin_unlock(ptl);
-				remainder = 0;
-				err = -ENOMEM;
-				break;
-			}
-		}
-
 		/*
 		 * If subpage information not requested, update counters
 		 * and skip the same_page loop below.
@@ -4405,7 +4392,22 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
 same_page:
 		if (pages) {
 			pages[i] = mem_map_offset(page, pfn_offset);
-			get_page(pages[i]);
+			/*
+			 * try_grab_page() should always succeed here, because:
+			 * a) we hold the ptl lock, and b) we've just checked
+			 * that the huge page is present in the page tables. If
+			 * the huge page is present, then the tail pages must
+			 * also be present. The ptl prevents the head page and
+			 * tail pages from being rearranged in any way. So this
+			 * page must be available at this point, unless the page
+			 * refcount overflowed:
+			 */
+			if (WARN_ON_ONCE(!try_grab_page(pages[i], flags))) {
+				spin_unlock(ptl);
+				remainder = 0;
+				err = -ENOMEM;
+				break;
+			}
 		}
 
 		if (vmas)
@@ -4965,6 +4967,12 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 	struct page *page = NULL;
 	spinlock_t *ptl;
 	pte_t pte;
+
+	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
+	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
+			 (FOLL_PIN | FOLL_GET)))
+		return NULL;
+
 retry:
 	ptl = pmd_lockptr(mm, pmd);
 	spin_lock(ptl);
@@ -4977,8 +4985,18 @@ follow_huge_pmd(struct mm_struct *mm, unsigned long address,
 	pte = huge_ptep_get((pte_t *)pmd);
 	if (pte_present(pte)) {
 		page = pmd_page(*pmd) + ((address & ~PMD_MASK) >> PAGE_SHIFT);
-		if (flags & FOLL_GET)
-			get_page(page);
+		/*
+		 * try_grab_page() should always succeed here, because: a) we
+		 * hold the pmd (ptl) lock, and b) we've just checked that the
+		 * huge pmd (head) page is present in the page tables. The ptl
+		 * prevents the head page and tail pages from being rearranged
+		 * in any way. So this page must be available at this point,
+		 * unless the page refcount overflowed:
+		 */
+		if (WARN_ON_ONCE(!try_grab_page(page, flags))) {
+			page = NULL;
+			goto out;
+		}
 	} else {
 		if (is_hugetlb_entry_migration(pte)) {
 			spin_unlock(ptl);
@@ -4999,7 +5017,7 @@ struct page * __weak
 follow_huge_pud(struct mm_struct *mm, unsigned long address,
 		pud_t *pud, int flags)
 {
-	if (flags & FOLL_GET)
+	if (flags & (FOLL_GET | FOLL_PIN))
 		return NULL;
 
 	return pte_page(*(pte_t *)pud) + ((address & ~PUD_MASK) >> PAGE_SHIFT);
@@ -5008,7 +5026,7 @@ follow_huge_pud(struct mm_struct *mm, unsigned long address,
 struct page * __weak
 follow_huge_pgd(struct mm_struct *mm, unsigned long address, pgd_t *pgd, int flags)
 {
-	if (flags & FOLL_GET)
+	if (flags & (FOLL_GET | FOLL_PIN))
 		return NULL;
 
 	return pte_page(*(pte_t *)pgd) + ((address & ~PGDIR_MASK) >> PAGE_SHIFT);