On 2/3/21 11:00 PM, John Hubbard wrote:

On 2/3/21 2:00 PM, Joao Martins wrote:

quoted

Add an helper that iterates over head pages in a list of pages. It
essentially counts the tails until the next page to process has a
different head that the current. This is going to be used by
unpin_user_pages() family of functions, to batch the head page refcount
updates once for all passed consecutive tail pages.

Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joao Martins <redacted>
---
  mm/gup.c | 29 +++++++++++++++++++++++++++++
  1 file changed, 29 insertions(+)

diff --git a/mm/gup.c b/mm/gup.c
index d68bcb482b11..4f88dcef39f2 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -215,6 +215,35 @@ void unpin_user_page(struct page *page)
  }
  EXPORT_SYMBOL(unpin_user_page);
  
+static inline unsigned int count_ntails(struct page **pages, unsigned long npages)

Silly naming nit: could we please name this function count_pagetails()? count_ntails
is a bit redundant, plus slightly less clear.

Hmm, pagetails is also a tiny bit redundant. Perhaps count_subpages() instead?

count_ntails is meant to be 'count number of tails' i.e. to align terminology with head +
tails which was also suggested over the other series.

quoted

+{
+	struct page *head = compound_head(pages[0]);
+	unsigned int ntails;
+
+	for (ntails = 1; ntails < npages; ntails++) {
+		if (compound_head(pages[ntails]) != head)
+			break;
+	}
+
+	return ntails;
+}
+
+static inline void compound_next(unsigned long i, unsigned long npages,
+				 struct page **list, struct page **head,
+				 unsigned int *ntails)
+{
+	if (i >= npages)
+		return;
+
+	*ntails = count_ntails(list + i, npages - i);
+	*head = compound_head(list[i]);
+}
+
+#define for_each_compound_head(i, list, npages, head, ntails) \

When using macros, which are dangerous in general, you have to worry about
things like name collisions. I really dislike that C has forced this unsafe
pattern upon us, but of course we are stuck with it, for iterator helpers.

/me nods

Given that we're stuck, you should probably use names such as __i, __list, etc,
in the the above #define. Otherwise you could stomp on existing variables.

Will do.

	Joao