Re: [PATCH 1/4] mm/gup: add compound page list iterator
From: John Hubbard <jhubbard@nvidia.com>
Date: 2021-02-03 23:03:13
Also in:
linux-mm, lkml
On 2/3/21 2:00 PM, Joao Martins wrote:
quoted hunk ↗ jump to hunk
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.
+{
+ 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. 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.
+ for (i = 0, compound_next(i, npages, list, &head, &ntails); \ + i < npages; i += ntails, \ + compound_next(i, npages, list, &head, &ntails)) + /** * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages * @pages: array of pages to be maybe marked dirty, and definitely released.
thanks, -- John Hubbard NVIDIA