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