On Wed, 2009-03-11 at 04:55 +0100, Nick Piggin wrote: > page_mkwrite is called with neither the page lock nor the ptl held. This > means a page can be concurrently truncated or invalidated out from underneath > it. Callers are supposed to prevent truncate races themselves, however > previously the only thing they can do in case they hit one is to raise a > SIGBUS. A sigbus is wrong for the case that the page has been invalidated > or truncated within i_size (eg. hole punched). Callers may also have to > perform memory allocations in this path, where again, SIGBUS would be wrong. > > The previous patch made it possible to properly specify errors. Convert > the generic buffer.c code and btrfs to return sane error values > (in the case of page removed from pagecache, VM_FAULT_NOPAGE will cause the > fault handler to exit without doing anything, and the fault will be retried > properly). > > This fixes core code, and converts btrfs as a template/example. All other > filesystems defining their own page_mkwrite should be fixed in a similar > manner. There appears to be another atomicity problem in the same area of code... The lack of locking between the call to ->page_mkwrite() and the subsequent call to set_page_dirty_balance() means that the filesystem may actually already have written out the page by the time you get round to calling set_page_dirty_balance(). How then is the filesystem supposed to guarantee that whatever structure it allocated in page_mkwrite() is still around when the page gets marked as dirty a second time? Trond -- 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
