KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxx> writes: > 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. I bet Andrea or Larry would remember the details. Cheers, Jeff -- 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