[PATCH v3 0/3] A few gup refactorings and documentation updates

[PATCH v3 0/3] A few gup refactorings and documentation updates

[John Hubbard]

Here is what's new for v3:

* Removed __maybe_unused from try_grab_compound_head
* Removed some unnecessary comparisons against NULL.
* Added Christoph's Reviewed-by tags to patches 2 and 3

Changes in v2:

* Changed refs to @refs, and added some more documentation as well.
* Completely removed try_get_page(). (I'm adding more people and lists
  to Cc, because of those call site changes.) * Reversed the logic in
try_grab_page() to make it a touch more
  readable.
* Rebased to linux-next (next-20210810).

Original v2 is here:
https://lore.kernel.org/r/20210811070542.3403116-1-jhubbard@nvidia.com

Here is the v1 cover letter, edited slightly to keep up with the latest
story.

While reviewing some of the other things going on around gup.c, I
noticed that the documentation was wrong for a few of the routines that
I wrote. And then I noticed that there was some significant code
duplication too. So this fixes those issues.

This is not entirely risk-free, but after looking closely at this, I
think it's actually a useful improvement, getting rid of the code
duplication here.

However, it is possible I've overlooked something. I did some local LTP
and other testing on an x86 test machine but failed to find any problems
yet.

Original v1 is here:
https://lore.kernel.org/r/20210808235018.1924918-1-jhubbard@nvidia.com



John Hubbard (3):
  mm/gup: documentation corrections for gup/pup
  mm/gup: small refactoring: simplify try_grab_page()
  mm/gup: Remove try_get_page(), call try_get_compound_head() directly

 arch/s390/mm/fault.c |  2 +-
 fs/pipe.c            |  2 +-
 include/linux/mm.h   | 14 ++------
 mm/gup.c             | 83 ++++++++++++++++++++++----------------------
 4 files changed, 47 insertions(+), 54 deletions(-)

-- 
2.32.0

[PATCH v3 1/3] mm/gup: documentation corrections for gup/pup

[John Hubbard]

The documentation for try_grab_compound_head() and try_grab_page() has
fallen a little out of date. Update and clarify a few points.

Also make it kerneldoc-correct, by adding @args documentation.

Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 mm/gup.c | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index 2630ed1bb4f4..52f08e3177e9 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -92,10 +92,17 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
 	return head;
 }
 
-/*
+/**
  * try_grab_compound_head() - attempt to elevate a page's refcount, by a
  * flags-dependent amount.
  *
+ * Even though the name includes "compound_head", this function is still
+ * appropriate for callers that have a non-compound @page to get.
+ *
+ * @page:  pointer to page to be grabbed
+ * @refs:  the value to (effectively) add to the page's refcount
+ * @flags: gup flags: these are the FOLL_* flag values.
+ *
  * "grab" names in this file mean, "look at flags to decide whether to use
  * FOLL_PIN or FOLL_GET behavior, when incrementing the page's refcount.
  *
@@ -103,8 +110,14 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
  * same time. (That's true throughout the get_user_pages*() and
  * pin_user_pages*() APIs.) Cases:
  *
- *    FOLL_GET: page's refcount will be incremented by 1.
- *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ *    FOLL_GET: page's refcount will be incremented by @refs.
+ *
+ *    FOLL_PIN on compound pages that are > two pages long: page's refcount will
+ *    be incremented by @refs, and page[2].hpage_pinned_refcount will be
+ *    incremented by @refs * GUP_PIN_COUNTING_BIAS.
+ *
+ *    FOLL_PIN on normal pages, or compound pages that are two pages long:
+ *    page's refcount will be incremented by @refs * GUP_PIN_COUNTING_BIAS.
  *
  * Return: head page (with refcount appropriately incremented) for success, or
  * NULL upon failure. If neither FOLL_GET nor FOLL_PIN was set, that's
@@ -141,6 +154,8 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page,
 		 *
 		 * However, be sure to *also* increment the normal page refcount
 		 * field at least once, so that the page really is pinned.
+		 * That's why the refcount from the earlier
+		 * try_get_compound_head() is left intact.
 		 */
 		if (hpage_pincount_available(page))
 			hpage_pincount_add(page, refs);
@@ -184,10 +199,8 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
  * @flags:   gup flags: these are the FOLL_* flag values.
  *
  * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
- * time. Cases:
- *
- *    FOLL_GET: page's refcount will be incremented by 1.
- *    FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
+ * time. Cases: please see the try_grab_compound_head() documentation, with
+ * "refs=1".
  *
  * Return: true for success, or if no action was required (if neither FOLL_PIN
  * nor FOLL_GET was set, nothing is done). False for failure: FOLL_GET or
-- 
2.32.0

[PATCH v3 2/3] mm/gup: small refactoring: simplify try_grab_page()

[John Hubbard]

try_grab_page() does the same thing as try_grab_compound_head(...,
refs=1, ...), just with a different API. So there is a lot of code
duplication there.

Change try_grab_page() to call try_grab_compound_head(), while keeping
the API contract identical for callers.

Also, now that try_grab_compound_head() always has a caller, remove the
__maybe_unused annotation.

Cc: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 include/linux/mm.h |  4 ++--
 mm/gup.c           | 35 +++++------------------------------
 2 files changed, 7 insertions(+), 32 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ce8fc0fd6d6e..ba985eaf3f19 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1204,8 +1204,8 @@ static inline void get_page(struct page *page)
 }
 
 bool __must_check try_grab_page(struct page *page, unsigned int flags);
-__maybe_unused struct page *try_grab_compound_head(struct page *page, int refs,
-						   unsigned int flags);
+struct page *try_grab_compound_head(struct page *page, int refs,
+				    unsigned int flags);
 
 
 static inline __must_check bool try_get_page(struct page *page)
diff --git a/mm/gup.c b/mm/gup.c
index 52f08e3177e9..886d6148d3d0 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -124,8 +124,8 @@ static inline struct page *try_get_compound_head(struct page *page, int refs)
  * considered failure, and furthermore, a likely bug in the caller, so a warning
  * is also emitted.
  */
-__maybe_unused struct page *try_grab_compound_head(struct page *page,
-						   int refs, unsigned int flags)
+struct page *try_grab_compound_head(struct page *page,
+				    int refs, unsigned int flags)
 {
 	if (flags & FOLL_GET)
 		return try_get_compound_head(page, refs);
@@ -208,35 +208,10 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags)
  */
 bool __must_check try_grab_page(struct page *page, unsigned int flags)
 {
-	WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == (FOLL_GET | FOLL_PIN));
+	if (!(flags & (FOLL_GET | FOLL_PIN)))
+		return true;
 
-	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;
-
-		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);
-
-		mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, 1);
-	}
-
-	return true;
+	return try_grab_compound_head(page, 1, flags);
 }
 
 /**
-- 
2.32.0

[PATCH v3 3/3] mm/gup: Remove try_get_page(), call try_get_compound_head() directly

[John Hubbard]

try_get_page() is very similar to try_get_compound_head(), and in fact
try_get_page() has fallen a little behind in terms of maintenance:
try_get_compound_head() handles speculative page references more
thoroughly.

There are only two try_get_page() callsites, so just call
try_get_compound_head() directly from those, and remove try_get_page()
entirely.

Also, seeing as how this changes try_get_compound_head() into a
non-static function, provide some kerneldoc documentation for it.

Cc: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---
 arch/s390/mm/fault.c |  2 +-
 fs/pipe.c            |  2 +-
 include/linux/mm.h   | 10 +---------
 mm/gup.c             | 21 +++++++++++++++++----
 4 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 212632d57db9..a834e4672f72 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -822,7 +822,7 @@ void do_secure_storage_access(struct pt_regs *regs)
 		break;
 	case KERNEL_FAULT:
 		page = phys_to_page(addr);
-		if (unlikely(!try_get_page(page)))
+		if (unlikely(!try_get_compound_head(page, 1)))
 			break;
 		rc = arch_make_page_accessible(page);
 		put_page(page);
diff --git a/fs/pipe.c b/fs/pipe.c
index 8e6ef62aeb1c..4394b80dc84f 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -191,7 +191,7 @@ EXPORT_SYMBOL(generic_pipe_buf_try_steal);
  */
 bool generic_pipe_buf_get(struct pipe_inode_info *pipe, struct pipe_buffer *buf)
 {
-	return try_get_page(buf->page);
+	return try_get_compound_head(buf->page, 1);
 }
 EXPORT_SYMBOL(generic_pipe_buf_get);
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index ba985eaf3f19..f5ab4fd6d48c 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1207,15 +1207,7 @@ bool __must_check try_grab_page(struct page *page, unsigned int flags);
 struct page *try_grab_compound_head(struct page *page, int refs,
 				    unsigned int flags);
 
-
-static inline __must_check bool try_get_page(struct page *page)
-{
-	page = compound_head(page);
-	if (WARN_ON_ONCE(page_ref_count(page) <= 0))
-		return false;
-	page_ref_inc(page);
-	return true;
-}
+struct page *try_get_compound_head(struct page *page, int refs);
 
 /**
  * folio_put - Decrement the reference count on a folio.
diff --git a/mm/gup.c b/mm/gup.c
index 886d6148d3d0..7a406d79bd2e 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -62,11 +62,24 @@ static void put_page_refs(struct page *page, int refs)
 	put_page(page);
 }
 
-/*
- * Return the compound head page with ref appropriately incremented,
- * or NULL if that failed.
+/**
+ * try_get_compound_head() - return the compound head page with refcount
+ * appropriately incremented, or NULL if that failed.
+ *
+ * This handles potential refcount overflow correctly. It also works correclty
+ * for various lockless get_user_pages()-related callers, due to the use of
+ * page_cache_add_speculative().
+ *
+ * Even though the name includes "compound_head", this function is still
+ * appropriate for callers that have a non-compound @page to get.
+ *
+ * @page:  pointer to page to be gotten
+ * @refs:  the value to add to the page's refcount
+ *
+ * Return: head page (with refcount appropriately incremented) for success, or
+ * NULL upon failure.
  */
-static inline struct page *try_get_compound_head(struct page *page, int refs)
+struct page *try_get_compound_head(struct page *page, int refs)
 {
 	struct page *head = compound_head(page);
 
-- 
2.32.0