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SubjectRe: freeze vs freezer
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On Thursday, 3 of January 2008, Nigel Cunningham wrote:
> Hi.
>
> Rafael J. Wysocki wrote:
> > On Wednesday, 2 of January 2008, Nigel Cunningham wrote:
> >> Pavel Machek wrote:
> >>>>>>>> So how do you handle threads that are blocked on I/O or a lock
> >>>>>>>> during the system freeze process, then?
> >>>>>>> We wait until they can continue.
> >>>>>> So if I have a process blocked on an unavilable NFS mount, I can't
> >>>>>> suspend?
> >>>>> That's correct, you can't.
> >>>>>
> >>>>> [And I know what you're going to say. ;-)]
> >>>> Why exactly does suspend/hibernation depend on "TASK_INTERRUPTIBLE"
> >>>> instead of a zero preempt_count()? Really what we should do is just
> >>>> iterate over all of the actual physical devices and tell each one
> >>>> "Block new IO requests preemptably, finish pending DMA, put the
> >>>> hardware in low-power mode, and prepare for suspend/hibernate". As
> >>>> long as each driver knows how to do those simple things we can have
> >>>> an entirely consistent kernel image for both suspend and for
> >>>> hibernation.
> >>> "each driver" means this is a lot of work. But yes, that is probably
> >>> way to go, and patch would be welcome.
> >> Yes, that does work. It's what I've done in my (preliminary) support for
> >> fuse.
> >
> > Hmm, can you please elaborate a bit?
>
> Sorry. I wasn't very unambiguous, was I? And I'm not sure now whether
> you're meaning "How does fuse support relate to freezing block devices?"
> or "What's this about fuse support?". Let me therefore seek to answer
> both questions:
>
> Higher level, I know (filesystems rather than block devices), but I was
> meaning the general concept of blocking new requests and completing
> existing ones worked fine for the supposedly impossible fuse support.
>
> Re fuse support, let me start by saying "I know this doesn't handle all
> situations, but I think it's a good enough proof-of-concept implementation".
>
> I added some simple hooks to the code for submitting new work to fuse
> threads.
>
> #define FUSE_MIGHT_FREEZE(superblock, desc) \
> do { \
> int printed = 0; \
> while(superblock->s_frozen != SB_UNFROZEN) { \
> if (!printed) { \
> printk("%d frozen in " desc ".\n", current->pid); \
> printed = 1; \
> } \
> try_to_freeze(); \
> yield(); \
> } \
> } while (0)
>
> On top of this, I made a (too simple at the moment) freeze_filesystems
> function which iterates through &super_blocks in reverse order, freezing
> fuse filesystems or ordinary ones. I say 'too simple' because it doesn't
> currently allow for the possibility of someone mounting (say) ext3 on
> fuse, but that would just be an extension of what's already done.
>
> The end result is:
>
> int freeze_processes(void)
> {
> int error;
>
> printk(KERN_INFO "Stopping fuse filesystems.\n");
> freeze_filesystems(FS_FREEZER_FUSE);
> freezer_state = FREEZER_FILESYSTEMS_FROZEN;
> printk(KERN_INFO "Freezing user space processes ... ");
> error = try_to_freeze_tasks(FREEZER_USER_SPACE);
> if (error)
> goto Exit;
> printk(KERN_INFO "done.\n");
>
> sys_sync();
> printk(KERN_INFO "Stopping normal filesystems.\n");
> freeze_filesystems(FS_FREEZER_NORMAL);
> freezer_state = FREEZER_USERSPACE_FROZEN;
> printk(KERN_INFO "Freezing remaining freezable tasks ... ");
> error = try_to_freeze_tasks(FREEZER_KERNEL_THREADS);
> if (error)
> goto Exit;
> printk(KERN_INFO "done.");
> freezer_state = FREEZER_FULLY_ON;
> Exit:
> BUG_ON(in_atomic());
> printk("\n");
> return error;
> }
>
> Sorry if that's more info than you wanted.

No, that's fine, thanks.

Greetings,
Rafael


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