On Thursday, 26 April 2007 22:16, Nigel Cunningham wrote: > Hi. > > On Thu, 2007-04-26 at 21:28 +0200, Rafael J. Wysocki wrote: > > On Thursday, 26 April 2007 18:10, Pekka Enberg wrote: > > > > > > On 4/26/2007, "Rafael J. Wysocki" <[EMAIL PROTECTED]> wrote: > > > > In principle, we could add suspend2 as an alternative (in analogy with > > > > the I/O > > > > schedulers, for example), but I think for this purpose it should be > > > > reviewed > > > > properly. > > > > > > Yeah, this makes sense. > > > > > > On 4/26/2007, "Rafael J. Wysocki" <[EMAIL PROTECTED]> wrote: > > > > There also is a real problem with how it uses the LRU pages. It > > > > _seems_ to > > > > work, but at least to me it seems to be potentially dangerous. > > > > > > I am new to suspend2 so can you please explain what exactly is dangerous > > > about it? > > > > After freezing tasks, it first saves the contents of the LRU pages, freezes > > devices and then uses the LRU pages for storing the suspend image (if more > > memory is needed, it's allocated, but that's irrelevant here). Now, we > > have no > > warranty that the LRU pages are not updated after we've saved their contents > > (first potential problem here). > > > > After the image has been created, we have to unfreeze devices and save the > > image. Now, we have no warranty that no one will be writing to the LRU > > pages > > that we have used to store the image, for whatever reasons known to him, so > > the > > image can potentially get corrupted while it's being saved. > > > > In principle, device drivers can do this and there are some kernel threads > > that > > also can do this (we don't freeze them, because they're needed for the image > > saving). > > > > The design is conceptually really really complicated and it makes strong > > assumptions about the behavior of different subsystems. While these > > assumptions _may_ be satisfied right now, we'd have to ensure the > > satisfaction > > of them in the future if suspend2 were merged. > > That's a good description of the issue, although I think _may_ and > _seems_ are stating things a bit more pessimistically than is > necessary.
I've used them to express my personal concerns. > You see, we need to remember that the pages which are saved separately > are LRU pages. Because userspace is frozen, their contents are going to > be static. The only possibilities for modifying them come from timer > routines, improperly frozen filesystems and device drivers. And kernel threads that we don't freeze deliberately. Currently, these are all worker threads, dm-related kernel threads and some others. > We have code to check that the LRU isn't changing, and I've only seen > one report of modifications to about 20 LRU pages. I haven't had the > time yet to chase down the cause, but hope to do so soon. I didn't say that would be common. If it had been, you'd have seen problems with it. To me the problem is the lack of warranty that it won't happen. > The general scheme has been working for four or five years - if there > was a fundamental issue, we would have found it by now. > > The scheme isn't complicated. Conceptually, it is complicated just because you're using the LRU. Greetings, Rafael - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
