Hi. On Thu, 2007-04-26 at 23:22 +0200, Rafael J. Wysocki wrote: > On Thursday, 26 April 2007 22:55, Nigel Cunningham wrote: > > Hi. > > > > On Thu, 2007-04-26 at 22:37 +0200, Rafael J. Wysocki wrote: > > > 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. > > > > Well, I'm willing to look at other ideas. I actually selected LRU > > because it was simple. Prior to this, we did have a try at just > > iterating over the pages of frozen processes, but it didn't yield enough > > pages to be viable. I wouldn't be surprised if hunting down the cause of > > these changing pages leads to doing the opposite - starting with LRU > > pages and then removing all the ones owned by processes. (Am I right in > > thinking the remainder would be anonymous pages? I must learn more mm > > inards :>). > > I think we can discuss that, and the other things too. I'm open to > cooperation. :-)
Great! Nigel
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