Re: set_page_dirty vs truncate

[Matthew Wilcox <willy@infradead.org>]

On Mon, Dec 21, 2020 at 03:12:57PM +0100, Jan Kara wrote:
> But overall even with GUP woes fixed up, set_page_dirty() called by a PUP
> user could still see already truncated page. So it has to deal with it.

Thanks!  That was really helpful.  We have a number of currently-buggy
filesystems which assume they can do inode = page->mapping->host without
checking that page->mapping is not NULL.

Anyway, since I'm changing the set_page_dirty signature for folios,
this feels like the right time to pass in the page's mapping.
__set_page_dirty() rechecks the mapping under the i_pages lock, so we
won't do anything inappropriate if the page has been truncated.

You can find the whole thing at
https://git.infradead.org/users/willy/pagecache.git/shortlog/refs/heads/folio

but the important bit is:

-       /* Set a page dirty.  Return true if this dirtied it */
-       int (*set_page_dirty)(struct page *page);
+       /* Set a folio dirty.  Return true if this dirtied it */
+       bool (*set_page_dirty)(struct address_space *, struct folio *);

I'm kind of tempted to rename it to ->dirty_folio(), but I'm also fine
with leaving it this way.