On Friday, 10 November 2006 00:32, Pavel Machek wrote: > Hi! > > > On Fri, Nov 10, 2006 at 12:11:46AM +0100, Pavel Machek wrote: > > > ? Not sure if I quite understand, but if dm breaks sync... something > > > is teribly wrong with dm. And we do simple sys_sync()... so I do not > > > think we have a problem. > > > > If you want to handle arbitrary kernel state, you might have a device-mapper > > device somewhere lower down the stack of devices that is queueing any I/O > > that reaches it. So anything waiting for I/O completion will wait until > > the dm process that suspended that device has finished whatever it is doing > > - and that might be a quick thing carried out by a userspace lvm tool, or > > a long thing carried out by an administrator using dmsetup. > > > > I'm guessing you need a way of detecting such state lower down the stack > > then optionally either aborting the operation telling the user it can't be > > done at present; waiting for however long it takes (perhaps for ever if > > the admin disappeared); or more probably skipping those devices on a > > 'best endeavours' basis. > > Okay, so you claim that sys_sync can stall, waiting for administator? > > In such case we can simply do one sys_sync() before we start freezing > userspace... or just more the only sys_sync() there. That way, admin > has chance to unlock his system. Well, this is a different story. My point is that if we call sys_sync() _anyway_ before calling freeze_filesystems(), then freeze_filesystems() is _safe_ (either the sys_sync() blocks, or it doesn't in which case freeze_filesystems() won't block either). This means, however, that we can leave the patch as is (well, with the minor fix I have already posted), for now, because it doesn't make things worse a bit, but: (a) it prevents xfs from being corrupted and (b) it prevents journaling filesystems in general from replaying journals after a failing resume. Still, there is a problem with the possibility of potential lock-up - either with the bdevs-freezing patch or without it - due to a suspended dm device down the stack and solving that is a _separate_ issue. Now if we use the userland suspend, there's no problem at all, I think, because s2disk calls sync() before it goes to suspend_system(), so the admin will have a chance to unclock the system and everything is fine and dandy (although it should be documented somewhere, IMHO). However, if the built-in swsusp is used, then well ... <looks> ... we can put a call to sys_sync() before prepare_processes() in pm_suspend_disk(). Greetings, Rafael -- You never change things by fighting the existing reality. R. Buckminster Fuller -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
