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? -- 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
