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Date: Mon, 18 Apr 2016 19:20:03 +0200 From: Florian Margaine <florian@...gaine.com> To: Eric Sandeen <sandeen@...deen.net>, Dave Chinner <david@...morbit.com> Cc: Alexander Viro <viro@...iv.linux.org.uk>, Jeff Layton <jlayton@...chiereds.net>, "J. Bruce Fields" <bfields@...ldses.org>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] fs: add the FIGETFROZEN ioctl call On Mon, 2016-04-18 at 11:20 -0400, Eric Sandeen wrote: > > On 4/14/16 10:17 PM, Dave Chinner wrote: > > On Thu, Apr 14, 2016 at 09:57:07AM +0200, Florian Margaine wrote: > > > This lets userland get the filesystem freezing status, aka > > > whether the > > > filesystem is frozen or not. This is so that an application can > > > know if > > > it should freeze the filesystem or if it isn't necessary when > > > taking a > > > snapshot. > > > > freezing nests, so there is no reason for avoiding a freeze when > > doing a snapshot. > > Sadly, no: > > # xfs_freeze -f /mnt/test > # xfs_freeze -f /mnt/test > xfs_freeze: cannot freeze filesystem at /mnt/test: Device or resource > busy > > It used to, but it was broken^Wchanged quite some time ago. I guess I can provide a patch for this. Silently making it a no-op if the FS is already frozen in ioctl_fsfreeze() should be good enough? > > > Indeed, if you don't wrap freeze/thaw around a > > snapshot, then if the fs is thawed while the snapshot is in > > progress > > then you are going to get a corrupt snapshot.... > > Yep. > > IMHO what really needs to happen is to fix freeze to allow nesting > again. > > A way to query freeze state might be nice, I think, but yeah, it's > racy, so you can't depend on it - but it might be useful in the "huh, > IO is failing, what's going on? Oh, it's frozen, ok" scenario... I guess my original use case was a bit contrived, but that or simply monitoring would be glad to have this method, indeed. > > But if you want to be sure your snapshot is OK even while others are > running concurrent snapshots, we need nested freezes to work again. > > -Eric > > > And, besides, polling for frozenness from userspace is inherently > > racy - by the time the syscall returns, the information may be > > incorrect, so you can't rely on it for decision making purposes in > > userspace. > > > > > +static int ioctl_fsgetfrozen(struct file *filp) > > > +{ > > > + struct super_block *sb = file_inode(filp)->i_sb; > > > + > > > + if (!capable(CAP_SYS_ADMIN)) > > > + return -EPERM; > > > + > > > + return sb->s_writers.frozen; > > > > This makes the internal freeze implementation states part of the > > userspace ABI. This needs an API that is separate from the > > internal > > implementation... > > > > Cheers, > > > > Dave. > > Regards, Florian
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