On Tue, Sep 23, 2014 at 05:03:22PM +0200, Jan Kara wrote:

->page_mkwrite() is used by filesystems to allocate blocks under a page
which is becoming writeably mmapped in some process' address space. This
allows a filesystem to return a page fault if there is not enough space
available, user exceeds quota or similar problem happens, rather than
silently discarding data later when writepage is called.

However VFS fails to call ->page_mkwrite() in all the cases where
filesystems need it when blocksize < pagesize. For example when
blocksize = 1024, pagesize = 4096 the following is problematic:
  ftruncate(fd, 0);
  pwrite(fd, buf, 1024, 0);
  map = mmap(NULL, 1024, PROT_WRITE, MAP_SHARED, fd, 0);
  map[0] = 'a';       ----> page_mkwrite() for index 0 is called
  ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */
  mremap(map, 1024, 10000, 0);
  map[4095] = 'a';    ----> no page_mkwrite() called

At the moment ->page_mkwrite() is called, filesystem can allocate only
one block for the page because i_size == 1024. Otherwise it would create
blocks beyond i_size which is generally undesirable. But later at
->writepage() time, we also need to store data at offset 4095 but we
don't have block allocated for it.

...

 
+#ifdef CONFIG_MMU
+/**
+ * block_create_hole - handle creation of a hole in a file
+ * @inode:	inode where the hole is created
+ * @from:	offset in bytes where the hole starts
+ * @to:		offset in bytes where the hole ends.

This function doesn't create holes.  It also manipulates page state,
not block state.  Probably could do with a better name, but I'm not
sure what a better name is - something like
pagecache_extend_isize(old_eof, new_eof)?

+void block_create_hole(struct inode *inode, loff_t from, loff_t to)
+{
+	int bsize = 1 << inode->i_blkbits;
+	loff_t rounded_from;
+	struct page *page;
+	pgoff_t index;
+
+	WARN_ON(!mutex_is_locked(&inode->i_mutex));
+	WARN_ON(to > inode->i_size);

We've already changed i_size, so shouldn't that be:

	WARN_ON(to != inode->i_size);

+
+	if (from >= to || bsize == PAGE_CACHE_SIZE)
+		return;
+	/* Currently last page will not have any hole block created? */
+	rounded_from = ALIGN(from, bsize);

That rounds down? or up? round_down/round_up are much better than
ALIGN() because they tell you exactly what rounding was intended...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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