On Wed 02-05-12 19:09:54, Nick Piggin wrote: > On 2 May 2012 18:17, Jan Kara <jack@xxxxxxx> wrote: > > On Wed 02-05-12 01:50:46, Nick Piggin wrote: > > >> KOSAKI-san is correct, I think. > >> > >> The race is something like this: > >> > >> DIO-read > >> page = get_user_pages() > >> fork() > >> COW(page) > >> touch(page) > >> DMA(page) > >> page_cache_release(page); > >> > >> So whether parent or child touches the page, determines who gets the > >> actual DMA target, and who gets the copy. > > OK, this is roughly what I understood from original threads as well. So > > if our buffer is page aligned and its size is page aligned, you would hit > > the corruption only if you do modify the buffer while IO to / from that buffer > > is in progress. And that would seem like a really bad programming practice > > anyway. So I still believe that having everything page size aligned will > > effectively remove the problem although I agree it does not aim at the core > > of it. > > I see what you mean. > > I'm not sure, though. For most apps it's bad practice I think. If you get into > realm of sophisticated, performance critical IO/storage managers, it would > not surprise me if such concurrent buffer modifications could be allowed. > We allow exactly such a thing in our pagecache layer. Although probably > those would be using shared mmaps for their buffer cache. > > I think it is safest to make a default policy of asking for IOs against private > cow-able mappings to be quiesced before fork, so there are no surprises > or reliance on COW details in the mm. Do you think? Yes, I agree that (and MADV_DONTFORK) is probably the best thing to have in documentation. Otherwise it's a bit too hairy... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
