Re: [PATCH 3/4] mm/gup: add a range variant of unpin_user_pages_dirty_lock()

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On 2/3/21 2:00 PM, Joao Martins wrote:
Add a unpin_user_page_range() API which takes a starting page
and how many consecutive pages we want to dirty.

Given that we won't be iterating on a list of changes, change
compound_next() to receive a bool, whether to calculate from the starting
page, or walk the page array. Finally add a separate iterator,

A bool arg is sometimes, but not always, a hint that you really just want
a separate set of routines. Below...

for_each_compound_range() that just operate in page ranges as opposed
to page array.

For users (like RDMA mr_dereg) where each sg represents a
contiguous set of pages, we're able to more efficiently unpin
pages without having to supply an array of pages much of what
happens today with unpin_user_pages().

Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
  include/linux/mm.h |  2 ++
  mm/gup.c           | 48 ++++++++++++++++++++++++++++++++++++++--------
  2 files changed, 42 insertions(+), 8 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index a608feb0d42e..b76063f7f18a 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1265,6 +1265,8 @@ static inline void put_page(struct page *page)
  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_page_range_dirty_lock(struct page *page, unsigned long npages,
+				      bool make_dirty);
  void unpin_user_pages(struct page **pages, unsigned long npages);
/**
diff --git a/mm/gup.c b/mm/gup.c
index 971a24b4b73f..1b57355d5033 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -215,11 +215,16 @@ void unpin_user_page(struct page *page)
  }
  EXPORT_SYMBOL(unpin_user_page);
-static inline unsigned int count_ntails(struct page **pages, unsigned long npages)
+static inline unsigned int count_ntails(struct page **pages,
+					unsigned long npages, bool range)
  {
-	struct page *head = compound_head(pages[0]);
+	struct page *page = pages[0], *head = compound_head(page);
  	unsigned int ntails;
+ if (range)
+		return (!PageCompound(head) || compound_order(head) <= 1) ? 1 :
+		   min_t(unsigned int, (head + compound_nr(head) - page), npages);

Here, you clearly should use a separate set of _range routines. Because you're basically
creating two different routines here! Keep it simple.

Once you're in a separate routine, you might feel more comfortable expanding that to
a more readable form, too:

	if (!PageCompound(head) || compound_order(head) <= 1)
		return 1;

	return min_t(unsigned int, (head + compound_nr(head) - page), npages);


thanks,
--
John Hubbard
NVIDIA

+
  	for (ntails = 1; ntails < npages; ntails++) {
  		if (compound_head(pages[ntails]) != head)
  			break;
@@ -229,20 +234,32 @@ static inline unsigned int count_ntails(struct page **pages, unsigned long npage
  }
static inline void compound_next(unsigned long i, unsigned long npages,
-				 struct page **list, struct page **head,
-				 unsigned int *ntails)
+				 struct page **list, bool range,
+				 struct page **head, unsigned int *ntails)
  {
+	struct page *p, **next = &p;
+
  	if (i >= npages)
  		return;
- *ntails = count_ntails(list + i, npages - i);
-	*head = compound_head(list[i]);
+	if (range)
+		*next = *list + i;
+	else
+		next = list + i;
+
+	*ntails = count_ntails(next, npages - i, range);
+	*head = compound_head(*next);
  }
+#define for_each_compound_range(i, list, npages, head, ntails) \
+	for (i = 0, compound_next(i, npages, list, true, &head, &ntails); \
+	     i < npages; i += ntails, \
+	     compound_next(i, npages, list, true,  &head, &ntails))
+
  #define for_each_compound_head(i, list, npages, head, ntails) \
-	for (i = 0, compound_next(i, npages, list, &head, &ntails); \
+	for (i = 0, compound_next(i, npages, list, false, &head, &ntails); \
  	     i < npages; i += ntails, \
-	     compound_next(i, npages, list, &head, &ntails))
+	     compound_next(i, npages, list, false,  &head, &ntails))
/**
   * unpin_user_pages_dirty_lock() - release and optionally dirty gup-pinned pages
@@ -306,6 +323,21 @@ void unpin_user_pages_dirty_lock(struct page **pages, unsigned long npages,
  }
  EXPORT_SYMBOL(unpin_user_pages_dirty_lock);
+void unpin_user_page_range_dirty_lock(struct page *page, unsigned long npages,
+				      bool make_dirty)
+{
+	unsigned long index;
+	struct page *head;
+	unsigned int ntails;
+
+	for_each_compound_range(index, &page, npages, head, ntails) {
+		if (make_dirty && !PageDirty(head))
+			set_page_dirty_lock(head);
+		put_compound_head(head, ntails, FOLL_PIN);
+	}
+}
+EXPORT_SYMBOL(unpin_user_page_range_dirty_lock);
+
  /**
   * unpin_user_pages() - release an array of gup-pinned pages.
   * @pages:  array of pages to be marked dirty and released.




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