On Mon, 28 Apr 2014 15:58:02 -0700 Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > On Mon, Apr 28, 2014 at 3:39 PM, Davidlohr Bueso <davidlohr@xxxxxx> wrote: > > > > Is this perhaps a KVM guest? fwiw I see CONFIG_KVM_ASYNC_PF=y which is a > > user of use_mm(). > > So I tried to look through these guys, and that was one of the ones I looked at. > > It's using use_mm(), but it's only called through schedule_work(). > Which *should* mean that it's in a kernel thread and > vmacache_valid_mm() will not be true. > > HOWEVER. > > The whole "we don't use the vma cache on kernel threads" does seem to > be a pretty fragile approach to the whole workqueue etc issue. I think > we always use a kernel thread for workqueue entries, but at the same > time I'm not 100% convinced that we should *rely* on that kind of > behavior. I don't think that it's necessarily fundamentally guaranteed > conceptually - I could see, for example, some user of "flush_work()" > deciding to run the work *synchronously* within the context of the > process that does the flushing. Very good point. > Now, I don't think we actually do that, but my point is that I think > it's a bit dangerous to just say "only kernel threads do use_mm(), and > work entries are always done by kernel threads, so let's disable vma > caching for kernel threads". It may be *true*, but it's a very > indirect kind of true. > > That's why I think we might be better off saying "let's just > invalidate the vmacache in use_mm(), and not care about who does it". > No subtle indirect logic about why the caching is safe in one context > but not another. > > But quite frankly, I grepped for things that set "tsk->mm", and apart > from clearing it on exit, the only uses I found was copy_mm() (which > does that vmacache_flush()) and use_mm(). And all the use_mm() cases > _seem_ to be in kernel threads, and that first BUG_ON() didn't have a > very complex call chain at all, just a regular page fault from udevd. unuse_mm() leaves current->mm at NULL so we'd hear about it pretty quickly if a user task was running use_mm/unuse_mm. Perhaps it's possible to do use_mm(new_mm); ... use_mm(old_mm); but nothing does that. > So it might just be some really nasty corruption totally unrelated to > the vmacache, and those preceding odd udevd-work and kdump faults > could be related. I think so. Maybe it's time to cook up a debug patch for Srivatsa to use? Dump the vma cache when the bug hits, or wire up some trace points. Or perhaps plain old printks - it seems to be happening pretty early in boot. Are there additional sanity checks we can perform at cache addition time? -- 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>