Relocking page in writepage

[Martin Jambor <jambormartin@gmail.com>]

Hi,

is it ok to do something like the following in aops->writepage (or in
writepages after calling clear_page_dirty_for_io(page) for that
matter):

        struct address_space mapping = page->mapping;
	page_cache_get(page);
	unlock_page(page);

        /* do something that might deadlock if page was locked */

	lock_page(page);
	page_cache_release(page);

	if (page->mapping != mapping) { /* truncated while doing the above */
		unlock_page(page);
		return 0;
	}

        /* continue as usual  (i.e. initiate writing, set writeback
and unlock) */

A few practical tests showed no problem but I thought it would be
better to ask anyway...
I know it isn't a nice thing to do but otherwise I would probably need
to implement some mechanism to remember the page I have locked and
never try to lock (or page_cache_grab) it again. Both are cumbersome
but this is easier, what do you think about it?

Thank you very much for any insight,

Martin Jambor