mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages

For huge pages (and in fact, any compound page), the GUP_PIN_COUNTING_BIAS
scheme tends to overflow too easily, each tail page increments the head
page->_refcount by GUP_PIN_COUNTING_BIAS (1024).  That limits the number
of huge pages that can be pinned.

This patch removes that limitation, by using an exact form of pin counting
for compound pages of order > 1.  The "order > 1" is required because this
approach uses the 3rd struct page in the compound page, and order 1
compound pages only have two pages, so that won't work there.

A new struct page field, hpage_pinned_refcount, has been added, replacing
a padding field in the union (so no new space is used).

This enhancement also has a useful side effect: huge pages and compound
pages (of order > 1) do not suffer from the "potential false positives"
problem that is discussed in the page_dma_pinned() comment block.  That is
because these compound pages have extra space for tracking things, so they
get exact pin counts instead of overloading page->_refcount.

Documentation/core-api/pin_user_pages.rst is updated accordingly.

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: John Hubbard <jhubbard@nvidia.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: Jérôme Glisse <jglisse@redhat.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/20200211001536.1027652-8-jhubbard@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 70 insertions, 8 deletions

diff --git a/mm/gup.c b/mm/gup.c
index ee4f14f108fe..6601df4c7682 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -29,6 +29,22 @@ struct follow_page_context {
 	unsigned int page_mask;
 };
 
+static void hpage_pincount_add(struct page *page, int refs)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+	atomic_add(refs, compound_pincount_ptr(page));
+}
+
+static void hpage_pincount_sub(struct page *page, int refs)
+{
+	VM_BUG_ON_PAGE(!hpage_pincount_available(page), page);
+	VM_BUG_ON_PAGE(page != compound_head(page), page);
+
+	atomic_sub(refs, compound_pincount_ptr(page));
+}
+
 /*
  * Return the compound head page with ref appropriately incremented,
  * or NULL if that failed.
@@ -70,8 +86,25 @@ static __maybe_unused struct page *try_grab_compound_head(struct page *page,
 	if (flags & FOLL_GET)
 		return try_get_compound_head(page, refs);
 	else if (flags & FOLL_PIN) {
-		refs *= GUP_PIN_COUNTING_BIAS;
-		return try_get_compound_head(page, refs);
+		/*
+		 * When pinning a compound page of order > 1 (which is what
+		 * hpage_pincount_available() checks for), use an exact count to
+		 * track it, via hpage_pincount_add/_sub().
+		 *
+		 * However, be sure to *also* increment the normal page refcount
+		 * field at least once, so that the page really is pinned.
+		 */
+		if (!hpage_pincount_available(page))
+			refs *= GUP_PIN_COUNTING_BIAS;
+
+		page = try_get_compound_head(page, refs);
+		if (!page)
+			return NULL;
+
+		if (hpage_pincount_available(page))
+			hpage_pincount_add(page, refs);
+
+		return page;
 	}
 
 	WARN_ON_ONCE(1);
@@ -106,12 +139,25 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 	if (flags & FOLL_GET)
 		return try_get_page(page);
 	else if (flags & FOLL_PIN) {
+		int refs = 1;
+
 		page = compound_head(page);
 
 		if (WARN_ON_ONCE(page_ref_count(page) <= 0))
 			return false;
 
-		page_ref_add(page, GUP_PIN_COUNTING_BIAS);
+		if (hpage_pincount_available(page))
+			hpage_pincount_add(page, 1);
+		else
+			refs = GUP_PIN_COUNTING_BIAS;
+
+		/*
+		 * Similar to try_grab_compound_head(): even if using the
+		 * hpage_pincount_add/_sub() routines, be sure to
+		 * *also* increment the normal page refcount field at least
+		 * once, so that the page really is pinned.
+		 */
+		page_ref_add(page, refs);
 	}
 
 	return true;
@@ -120,12 +166,17 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags)
 #ifdef CONFIG_DEV_PAGEMAP_OPS
 static bool __unpin_devmap_managed_user_page(struct page *page)
 {
-	int count;
+	int count, refs = 1;
 
 	if (!page_is_devmap_managed(page))
 		return false;
 
-	count = page_ref_sub_return(page, GUP_PIN_COUNTING_BIAS);
+	if (hpage_pincount_available(page))
+		hpage_pincount_sub(page, 1);
+	else
+		refs = GUP_PIN_COUNTING_BIAS;
+
+	count = page_ref_sub_return(page, refs);
 
 	/*
 	 * devmap page refcounts are 1-based, rather than 0-based: if
@@ -157,6 +208,8 @@ static bool __unpin_devmap_managed_user_page(struct page *page)
  */
 void unpin_user_page(struct page *page)
 {
+	int refs = 1;
+
 	page = compound_head(page);
 
 	/*
@@ -168,7 +221,12 @@ void unpin_user_page(struct page *page)
 	if (__unpin_devmap_managed_user_page(page))
 		return;
 
-	if (page_ref_sub_and_test(page, GUP_PIN_COUNTING_BIAS))
+	if (hpage_pincount_available(page))
+		hpage_pincount_sub(page, 1);
+	else
+		refs = GUP_PIN_COUNTING_BIAS;
+
+	if (page_ref_sub_and_test(page, refs))
 		__put_page(page);
 }
 EXPORT_SYMBOL(unpin_user_page);
@@ -1955,8 +2013,12 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
 
 static void put_compound_head(struct page *page, int refs, unsigned int flags)
 {
-	if (flags & FOLL_PIN)
-		refs *= GUP_PIN_COUNTING_BIAS;
+	if (flags & FOLL_PIN) {
+		if (hpage_pincount_available(page))
+			hpage_pincount_sub(page, refs);
+		else
+			refs *= GUP_PIN_COUNTING_BIAS;
+	}
 
 	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);
 	/*