Re: [PATCH block/for-next] writeback: add tracepoints for cgroup foreign writebacks

[Tejun Heo <tj@kernel.org>]

Hello, Jan.

On Fri, Aug 30, 2019 at 06:42:11PM +0200, Jan Kara wrote:
> Well, but if you look at __set_page_dirty_nobuffers() it is careful. It
> does:
> 
> struct address_space *mapping = page_mapping(page);
> 
> if (!mapping) {
> 	bail
> }
> ... use mapping
> 
> Exactly because page->mapping can become NULL under your hands if you don't
> hold page lock. So I think you either need something similar in your
> tracepoint or handle this in the caller.

So, account_page_dirtied() is called from two places.

__set_page_dirty() and __set_page_dirty_nobuffers().  The following is
from the latter.

	lock_page_memcg(page);
	if (!TestSetPageDirty(page)) {
		struct address_space *mapping = page_mapping(page);
		...

		if (!mapping) {
			unlock_page_memcg(page);
			return 1;
		}

		xa_lock_irqsave(&mapping->i_pages, flags);
		BUG_ON(page_mapping(page) != mapping);
		WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
		account_page_dirtied(page, mapping);
		...

If I'm reading it right, it's saying that at this point if mapping
exists after setting page dirty, it must not change while locking
i_pages.

__set_page_dirty_nobuffers() is more brief but seems to be making the
same assumption.

	xa_lock_irqsave(&mapping->i_pages, flags);
	if (page->mapping) {	/* Race with truncate? */
		WARN_ON_ONCE(warn && !PageUptodate(page));
		account_page_dirtied(page, mapping);
		__xa_set_mark(&mapping->i_pages, page_index(page),
				PAGECACHE_TAG_DIRTY);
	}
	xa_unlock_irqrestore(&mapping->i_pages, flags);

Both are clearly assuming that once i_pages is locked, mapping can't
change.  So, inside account_page_dirtied(), mapping clearly can't
change.  The TP in question - track_foreign_dirty - is invoked from
mem_cgroup_track_foreign_dirty() which is only called from
account_page_dirty(), so I'm failing to see how mapping would change
there.

Thanks.

-- 
tejun