[v5,03/28] mm: Add set/end/wait functions for PG_private_2

Commit Message

David Howells March 23, 2021, 10:18 p.m. UTC

Add three functions to manipulate PG_private_2:

 (*) set_page_private_2() - Set the flag and take an appropriate reference
     on the flagged page.

 (*) end_page_private_2() - Clear the flag, drop the reference and wake up
     any waiters, somewhat analogously with end_page_writeback().

 (*) wait_on_page_private_2() - Wait for the flag to be cleared.

Wrappers will need to be placed in the netfs lib header in the patch that
adds that.

[This implements a suggestion by Linus[1] to not mix the terminology of
 PG_private_2 and PG_fscache in the mm core function]

Changes:
v5:
- Add set and end functions, calling the end function end rather than
  unlock[3].
- Keep a ref on the page when PG_private_2 is set[4][5].

v4:
- Remove extern from the declaration[2].

Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
cc: Matthew Wilcox (Oracle) <[e-mail]>
cc: Alexander Viro <[e-mail]>
cc: Christoph Hellwig <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]/ # v1
Link: https://lore.kernel.org/r/CAHk-=wjgA-74ddehziVk=[e-mail]/ [1]
Link: https://lore.kernel.org/r/[e-mail]/ [2]
Link: https://lore.kernel.org/r/[e-mail]/ # v3
Link: https://lore.kernel.org/r/[e-mail] # v4
Link: https://lore.kernel.org/r/[e-mail] [3]
Link: https://lore.kernel.org/r/CAHk-=wh+2gbF7XEjYc=HV9w_2uVzVf7vs60BPz0gFA=[e-mail]/ [4]
Link: https://lore.kernel.org/r/CAHk-=[e-mail]/ [5]
---

 include/linux/pagemap.h |   19 +++++++++++++++
 mm/filemap.c            |   59 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 78 insertions(+)

Patch

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 444155ae56c0..da5c38864037 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -689,6 +689,25 @@  void wait_for_stable_page(struct page *page);
 
 void page_endio(struct page *page, bool is_write, int err);
 
+/**
+ * set_page_private_2 - Set PG_private_2 on a page and take a ref
+ * @page: The page.
+ *
+ * Set the PG_private_2 flag on a page and take the reference needed for the VM
+ * to handle its lifetime correctly.  This sets the flag and takes the
+ * reference unconditionally, so care must be taken not to set the flag again
+ * if it's already set.
+ */
+static inline void set_page_private_2(struct page *page)
+{
+	get_page(page);
+	SetPagePrivate2(page);
+}
+
+void end_page_private_2(struct page *page);
+void wait_on_page_private_2(struct page *page);
+int wait_on_page_private_2_killable(struct page *page);
+
 /*
  * Add an arbitrary waiter to a page's wait queue
  */
diff --git a/mm/filemap.c b/mm/filemap.c
index 43700480d897..788b71e8a72d 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1432,6 +1432,65 @@  void unlock_page(struct page *page)
 }
 EXPORT_SYMBOL(unlock_page);
 
+/**
+ * end_page_private_2 - Clear PG_private_2 and release any waiters
+ * @page: The page
+ *
+ * Clear the PG_private_2 bit on a page and wake up any sleepers waiting for
+ * this.  The page ref held for PG_private_2 being set is released.
+ *
+ * This is, for example, used when a netfs page is being written to a local
+ * disk cache, thereby allowing writes to the cache for the same page to be
+ * serialised.
+ */
+void end_page_private_2(struct page *page)
+{
+	page = compound_head(page);
+	VM_BUG_ON_PAGE(!PagePrivate2(page), page);
+	clear_bit_unlock(PG_private_2, &page->flags);
+	wake_up_page_bit(page, PG_private_2);
+	put_page(page);
+}
+EXPORT_SYMBOL(end_page_private_2);
+
+/**
+ * wait_on_page_private_2 - Wait for PG_private_2 to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_private_2 (aka PG_fscache) to be cleared on a page.
+ */
+void wait_on_page_private_2(struct page *page)
+{
+	while (PagePrivate2(page))
+		wait_on_page_bit(page, PG_private_2);
+}
+EXPORT_SYMBOL(wait_on_page_private_2);
+
+/**
+ * wait_on_page_private_2_killable - Wait for PG_private_2 to be cleared on a page
+ * @page: The page to wait on
+ *
+ * Wait for PG_private_2 (aka PG_fscache) to be cleared on a page or until a
+ * fatal signal is received by the calling task.
+ *
+ * Return:
+ * - 0 if successful.
+ * - -EINTR if a fatal signal was encountered.
+ */
+int wait_on_page_private_2_killable(struct page *page)
+{
+	int ret = 0;
+
+	while (PagePrivate2(page)) {
+		ret = wait_on_page_bit_killable(page, PG_private_2);
+		if (ret < 0)
+			break;
+	}
+
+	return ret;
+}
+EXPORT_SYMBOL(wait_on_page_private_2_killable);
+
 /**
  * end_page_writeback - end writeback against a page
  * @page: the page