linux-kernel - Re: [PATCH 0/6] power: wire-up filesystem freeze/thaw with suspend/resume

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Message-ID: <s6rnz3ysjlu3rp6m56vua3vnlj53hbgxbbe3nj7v2ib5fg4l2i@py4pkvsgk2lr>
Date: Tue, 1 Apr 2025 11:32:49 +0200
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, jack@...e.cz, rafael@...nel.org, 
	Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org, 
	James Bottomley <James.Bottomley@...senpartnership.com>, mcgrof@...nel.org, hch@...radead.org, david@...morbit.com, 
	djwong@...nel.org, pavel@...nel.org, peterz@...radead.org, mingo@...hat.com, 
	will@...nel.org, boqun.feng@...il.com
Subject: Re: [PATCH 0/6] power: wire-up filesystem freeze/thaw with
 suspend/resume

On Tue 01-04-25 02:32:45, Christian Brauner wrote:
> The whole shebang can also be found at:
> https://web.git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=work.freeze
> 
> I know nothing about power or hibernation. I've tested it as best as I
> could. Works for me (TM).
> 
> I need to catch some actual sleep now...
> 
> ---
> 
> Now all the pieces are in place to actually allow the power subsystem to
> freeze/thaw filesystems during suspend/resume. Filesystems are only
> frozen and thawed if the power subsystem does actually own the freeze.
> 
> Othwerwise it risks thawing filesystems it didn't own. This could be
> done differently be e.g., keeping the filesystems that were actually
> frozen on a list and then unfreezing them from that list. This is
> disgustingly unclean though and reeks of an ugly hack.
> 
> If the filesystem is already frozen by the time we've frozen all
> userspace processes we don't care to freeze it again. That's userspace's
> job once the process resumes. We only actually freeze filesystems if we
> absolutely have to and we ignore other failures to freeze.

Hum, I don't follow here. I supposed we'll use FREEZE_MAY_NEST |
FREEZE_HOLDER_KERNEL for freezing from power subsystem. As far as I
remember we have specifically designed nesting of freeze counters so that
this way power subsystem can be sure freezing succeeds even if the
filesystem is already frozen (by userspace or the kernel) and similarly
power subsystem cannot thaw a filesystem frozen by somebody else. It will
just drop its freeze refcount... What am I missing?

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR