[PATCH 55/64] afs: Handle len being extending over page end in write_begin/write_end

[David Howells]

With transparent huge pages, in the future, write_begin() and write_end()
may be passed a length parameter that, in combination with the offset into
the page, exceeds the length of that page.  This allows
grab_cache_page_write_begin() to better choose the size of THP to allocate.

Fix afs's functions to handle this by trimming the length as needed after
the page has been allocated.

[Removed the now-unnecessary index var; spotted by kernel test robot]

Fixes: e1b1240c1ff5 ("netfs: Add write_begin helper")
Reported-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
Acked-by: Jeff Layton <jlayton@xxxxxxxxxx>
cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
cc: linux-afs@xxxxxxxxxxxxxxxxxxx
Link: https://lore.kernel.org/r/162367681795.460125.11729955608839747375.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1
---

 fs/afs/write.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/afs/write.c b/fs/afs/write.c
index ca4909baf5e6..8e4e87d66855 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -25,7 +25,8 @@ int afs_set_page_dirty(struct page *page)
 }
 
 /*
- * prepare to perform part of a write to a page
+ * Prepare to perform part of a write to a page.  Note that len may extend
+ * beyond the end of the page.
  */
 int afs_write_begin(struct file *file, struct address_space *mapping,
 		    loff_t pos, unsigned len, unsigned flags,
@@ -36,7 +37,6 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 	unsigned long priv;
 	unsigned f, from;
 	unsigned t, to;
-	pgoff_t index;
 	int ret;
 
 	_enter("{%llx:%llu},%llx,%x",
@@ -51,8 +51,8 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 	if (ret < 0)
 		return ret;
 
-	index = folio_index(folio);
-	from = pos - index * PAGE_SIZE;
+	from = offset_in_folio(folio, pos);
+	len = min_t(size_t, len, folio_size(folio) - from);
 	to = from + len;
 
 try_again:
@@ -103,7 +103,8 @@ int afs_write_begin(struct file *file, struct address_space *mapping,
 }
 
 /*
- * finalise part of a write to a page
+ * Finalise part of a write to a page.  Note that len may extend beyond the end
+ * of the page.
  */
 int afs_write_end(struct file *file, struct address_space *mapping,
 		  loff_t pos, unsigned len, unsigned copied,