linux-kernel - Re: [PATCH] fs: allow nesting with FREEZE

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Message-ID: <Z_OWJqiOyVPWGq1a@infradead.org>
Date: Mon, 7 Apr 2025 02:08:54 -0700
From: Christoph Hellwig <hch@...radead.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.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,
	rafael@...nel.org, djwong@...nel.org, pavel@...nel.org,
	peterz@...radead.org, mingo@...hat.com, will@...nel.org,
	boqun.feng@...il.com
Subject: Re: [PATCH] fs: allow nesting with FREEZE_EXCL

On Fri, Apr 04, 2025 at 12:24:09PM +0200, Christian Brauner wrote:
> If hibernation races with filesystem freezing (e.g. DM reconfiguration),
> then hibernation need not freeze a filesystem because it's already
> frozen but userspace may thaw the filesystem before hibernation actually
> happens.
> 
> If the race happens the other way around, DM reconfiguration may
> unexpectedly fail with EBUSY.
> 
> So allow FREEZE_EXCL to nest with other holders. An exclusive freezer
> cannot be undone by any of the other concurrent freezers.

What is FREEZE_EXCL?  I can't find it anywhere including in linux-next.