On Mon, Mar 24, 2014 at 06:59:30PM +0800, Gu Zheng wrote: > As the page migration framework holds lock_page() to protect the pages > (both old and new) while migrating, so while the page migrating, both > of old page and new page are locked. And the aio context teardown > routine will call *truncate*(in put_aio_ring_file()) to truncate > pagecache which needs to acquire page_lock() for each page one by one. > So there is a native mutual exclusion between *migrate page* v.s. truncate(). > > If put_aio_ring_file() is called at first of the context teardown flow > (aio_free_ring). Then, page migration and ctx freeing will have mutual > execution guarded by lock_page() v.s. truncate(). Once a page is removed > from radix-tree, it will not be migrated. On the other hand, the context > can not be freed while the page migraiton are ongoing. Sorry, but your change to remove the taking of ->private_lock in put_aio_ring_file() is not safe. If a malicious user reinstantiates any pages in the ring buffer's mmaping, there is nothing protecting the system against incoherent accesses of ->ring_pages. One possible way of making this occur would be to use mremap() to expand the size of the mapping or move it to a different location in the user process' address space. Yes, it's a tiny race, but it's possible. There is absolutely no reason to remove this locking -- ring teardown is hardly a performance sensitive code path. I'm going to stick with my approach instead. -ben -- "Thought is the essence of where you are now." -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
