linux-kernel - [PATCH v2 2/4] fs: allow all writers to be frozen

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Message-ID: <20250402-work-freeze-v2-2-6719a97b52ac@kernel.org>
Date: Wed,  2 Apr 2025 16:07:32 +0200
From: Christian Brauner <brauner@...nel.org>
To: linux-fsdevel@...r.kernel.org,
	jack@...e.cz
Cc: Christian Brauner <brauner@...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,
	rafael@...nel.org,
	djwong@...nel.org,
	pavel@...nel.org,
	peterz@...radead.org,
	mingo@...hat.com,
	will@...nel.org,
	boqun.feng@...il.com
Subject: [PATCH v2 2/4] fs: allow all writers to be frozen

During freeze/thaw we need to be able to freeze all writers during
suspend/hibernate. Otherwise tasks such as systemd-journald that mmap a
file and write to it will not be frozen after we've already frozen the
filesystem.

This has some risk of not being able to freeze processes in case a
process has acquired SB_FREEZE_PAGEFAULT under mmap_sem or
SB_FREEZE_INTERNAL under some other filesytem specific lock. If the
filesystem is frozen, a task can block on the frozen filesystem with
e.g., mmap_sem held. If some other task then blocks on grabbing that
mmap_sem, hibernation ill fail because it is unable to hibernate a task
holding mmap_sem. This could be fixed by making a range of filesystem
related locks use freezable sleeping. That's impractical and not
warranted just for suspend/hibernate. Assume that this is an infrequent
problem and we've given userspace a way to skip filesystem freezing
through a sysfs file.

Signed-off-by: Christian Brauner <brauner@...nel.org>
---
 include/linux/fs.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index b379a46b5576..1edcba3cd68e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1781,8 +1781,7 @@ static inline void __sb_end_write(struct super_block *sb, int level)

 static inline void __sb_start_write(struct super_block *sb, int level)
 {
-	percpu_down_read_freezable(sb->s_writers.rw_sem + level - 1,
-				   level == SB_FREEZE_WRITE);
+	percpu_down_read_freezable(sb->s_writers.rw_sem + level - 1, true);
 }

 static inline bool __sb_start_write_trylock(struct super_block *sb, int level)

-- 
2.47.2