Re: [PATCH 00/11] fs: use freeze_fs on suspend/hibernate

[Dave Chinner]

On Fri, Dec 01, 2017 at 02:05:44PM -0500, Jeff Layton wrote:
> On Thu, 2017-11-30 at 17:41 +0100, Jiri Kosina wrote:
> > On Fri, 1 Dec 2017, Yu Chen wrote:
> > 
> > > BTW, is nfs able to be included in this set? I also encountered a 
> > > freeze() failure due to nfs access during that stage recently.
> > 
> > The freezer usage in NFS is magnitudes more complicated, so it makes sense 
> > to first go after the lower hanging fruit to figure out the viability of 
> > the whole aproach in practice.
> > 
> 
> Agreed that we should do this in stages. It doesn't help that freezer
> handling in the client is a bit of a mess at this point...
> 
> At a high level for NFS, I think we need to have freeze_fs make the RPC
> engine "park" newly issued RPCs for that fs' client onto a
> rpc_wait_queue. Any RPC that has already been sent however, we need to
> wait for a reply.
> 
> Once everything is quiesced we can return and call it frozen.
> unfreeze_fs can then just have the engine stop parking RPCs and wake up
> the waitq.

That seems pretty reasonable. freezing is expected to take a bit of
time to run - local filesystems can do a fair bit of IO draining
queues, inflight operations and bringing the journal into a
consistent state on disk before declaring the filesystem is frozen.

> That should be enough to make suspend and resume work more reliably. If,
> however, you're interested in making the cgroup freezer also work, then
> we may need to do a bit more work to ensure that we don't end up with
> frozen tasks squatting on VFS locks.

None of the existing freezing code gives those guarantees. In fact,
freezing a filesystem pretty much guarantees the opposite - that
tasks *will freeze when holding VFS locks* - and so the cgroup
freezer is broken by design if it requires tasks to be frozen
without holding any VFS/filesystem lock context. So I wouldn't
really worry about the cgroup freezer....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx