On Tue, May 1, 2012 at 11:11 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: > KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> writes: > >>> Hello, >>> >>> Thank you revisit this. But as far as my remember is correct, this issue is NOT >>> unaligned access issue. It's just get_user_pages(_fast) vs fork race issue. i.e. >>> DIRECT_IO w/ multi thread process should not use fork(). >> >> The problem is, fork (and its COW logic) assume new access makes cow break, >> But page table protection can't detect a DMA write. Therefore DIO may override >> shared page data. > > Hm, I've only seen this with misaligned or multiple sub-page-sized reads > in the same page. AFAIR, aligned, page-sized I/O does not get split. > But, I could be wrong... If my remember is correct, the reproducer of past thread is misleading. dma_thread.c in http://lkml.indiana.edu/hypermail/linux/kernel/0903.1/01498.html has align parameter. But it doesn't only change align. Because of, every worker thread read 4K (pagesize), then - when offset is page aligned -> every page is accessed from only one worker - when offset is not page aligned -> every page is accessed from two workers But I don't remember why two threads are important things. hmm.. I'm looking into the code a while. Please don't 100% trust me. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href