On Thu, Dec 13, 2012 at 11:27 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: > On Thu, Dec 13, 2012 at 09:49:43PM -0800, Andy Lutomirski wrote: >> This is a serious cause of mmap_sem contention. MAP_POPULATE >> and MCL_FUTURE, in particular, are disastrous in multithreaded programs. >> >> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx> >> --- >> >> Sensible people use anonymous mappings. I write kernel patches :) >> >> I'm not entirely thrilled by the aesthetics of this patch. The MAP_POPULATE case >> could also be improved by doing it without any lock at all. This is still a big >> improvement, though. > > Wait a minute. get_user_pages() relies on ->mmap_sem being held. Unless > I'm seriously misreading your patch it removes that protection. And yes, > I'm aware of execve-related exception; it's in special circumstances - > bprm->mm is guaranteed to be not shared (and we need to rearchitect that > area anyway, but that's a separate story). Unless I completely screwed up the patch, ->mmap_sem is still held for read (it's downgraded from write). It's just not held for write anymore. --Andy -- 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/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>