set_page_dirty vs set_page_dirty_lock

["Michael S. Tsirkin" <mst@mellanox.co.il>]

Hi!
The comment at set_page_dirty_lock says:

/*
 * set_page_dirty() is racy if the caller has no reference against
 * page->mapping->host, and if the page is unlocked.  This is because another
 * CPU could truncate the page off the mapping and then free the mapping.
 *
 * Usually, the page _is_ locked, or the caller is a user-space process which
 * holds a reference on the inode by having an open file.
 *
 * In other cases, the page should be locked before running set_page_dirty().
 */

Still, I wander whether it might be OK to use set_page_dirty
in another case - if I previously got a reference to the page
with get_user_pages?
The page wouldnt be written back in this case, would it?
What if I'm in the middle of a system call?

Thanks,

-- 
MST