Hi Ben, On 03/24/2014 09:20 PM, Benjamin LaHaise wrote: > 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. OK, you can go ahead via your approach, but I'll hold the reservation about the issue you mentioned above before I find out it clearly. BTW, please also send it to the 3.12.y and 3.13.y stable tree once it is merged into Linus' tree. Thanks, Gu > > -ben -- 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
