Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2

David Chinner
2007-03-19T08:13:16Z

On Mon, Mar 19, 2007 at 05:37:03PM +1100, Nick Piggin wrote:
> David Chinner wrote:
> >
> >+/*
> >+ * block_page_mkwrite() is not allowed to change the file size as it gets
> >+ * called from a page fault handler when a page is first dirtied. Hence
> >we must
> >+ * be careful to check for EOF conditions here. We set the page up
> >correctly
> >+ * for a written page which means we get ENOSPC checking when writing into
> >+ * holes and correct delalloc and unwritten extent mapping on filesystems
> >that
> >+ * support these features.
> >+ *
> >+ * We are not allowed to take the i_mutex here so we have to play games to
> >+ * protect against truncate races as the page could now be beyond EOF.
> >Because
> >+ * vmtruncate() writes the inode size before removing pages, once we have
> >the
> >+ * page lock we can determine safely if the page is beyond EOF. If it is
> >not
> >+ * beyond EOF, then the page is guaranteed safe against truncation until
> >we
> >+ * unlock the page.
> >+ */
> >+int
> >+block_page_mkwrite(struct vm_area_struct *vma, struct page *page,
> >+ get_block_t get_block)
> >+{
> >+ struct inode *inode = vma->vm_file->f_path.dentry->d_inode;
> >+ unsigned long end;
> >+ loff_t size;
> >+ int ret = -EINVAL;
> >+
> >+ lock_page(page);
> >+ size = i_size_read(inode);
> >+ if ((page->mapping != inode->i_mapping) ||
> >+ ((page->index << PAGE_CACHE_SHIFT) > size)) {
> >+ /* page got truncated out from underneath us */
> >+ goto out_unlock;
> >+ }
>
> I see your explanation above, but I still don't see why this can't
> just follow the conventional if (!page->mapping) check for truncation.
> If the test happens to be performed after truncate concurrently
> decreases i_size, then the blocks are going to get truncated by the
> truncate afterwards anyway.

We have to read the inode size in the normal case so that we know if
the page is at EOF and is a partial page so we don't allocate past EOF in
block_prepare_write(). Hence it seems like a no-brainer to me to check
and error out on a page that we *know* is beyond EOF.

I can drop the check if you see no value in it - I just don't
like the idea of ignoring obvious boundary condition violations...

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group