Re: [PATCH v3] ceph: fix write_begin optimization when write is beyond EOF

On Sat, Jun 12, 2021 at 02:35:31PM -0400, Jeff Layton wrote:

+/**

• • prep_noread_page - prep a page for writing without reading first
• • @page: page being prepared
• • @pos: starting position for the write
• • @len: length of write
• • In some cases we don't need to read at all:
• • • full page write
• • • file is currently zero-length
• • • write that lies in a page that is completely beyond EOF
• • • write that covers the the page from start to EOF or beyond it
• • If any of these criteria are met, then zero out the unwritten parts
• • of the page and return true. Otherwise, return false.
• */

+static bool prep_noread_page(struct page *page, loff_t pos, unsigned int len) +{

• struct inode *inode = page->mapping->host;
• loff_t i_size = i_size_read(inode);
• pgoff_t index = pos / PAGE_SIZE;
• int pos_in_page = pos & ~PAGE_MASK;

Like the helper. A couple of minor tweaks ...

size_t offset = offset_in_page(pos);

• /* full page write */
• if (pos_in_page == 0 && len == PAGE_SIZE)

• goto zero_out;
  

At some point, we're going to need to pass the full len to ->write_begin, so that we can decide whether it's worth allocating more than a single page. Could you make 'len' here size_t, and check for len >= PAGE_SIZE?

(with the current code, the offset of 0 is a redundant check, but I'd rather see this future-proofed).