linux-kernel - Re: [PATCH v2 0/6] Extend freeze support to suspend and hibernate

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Message-ID: <20250401-vorstadt-natur-38eb98783008@brauner>
Date: Tue, 1 Apr 2025 01:33:44 +0200
From: Christian Brauner <brauner@...nel.org>
To: Jan Kara <jack@...e.cz>
Cc: James Bottomley <James.Bottomley@...senpartnership.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 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 v2 0/6] Extend freeze support to suspend and hibernate

On Mon, Mar 31, 2025 at 12:36:27PM +0200, Jan Kara wrote:
> On Sat 29-03-25 13:02:32, James Bottomley wrote:
> > On Sat, 2025-03-29 at 10:04 -0400, James Bottomley wrote:
> > > On Sat, 2025-03-29 at 09:42 +0100, Christian Brauner wrote:
> > > > Add the necessary infrastructure changes to support freezing for
> > > > suspend and hibernate.
> > > > 
> > > > Just got back from LSFMM. So still jetlagged and likelihood of bugs
> > > > increased. This should all that's needed to wire up power.
> > > > 
> > > > This will be in vfs-6.16.super shortly.
> > > > 
> > > > ---
> > > > Changes in v2:
> > > > - Don't grab reference in the iterator make that a requirement for
> > > > the callers that need custom behavior.
> > > > - Link to v1:
> > > > https://lore.kernel.org/r/20250328-work-freeze-v1-0-a2c3a6b0e7a6@kernel.org
> > > 
> > > Given I've been a bit quiet on this, I thought I'd better explain
> > > what's going on: I do have these built, but I made the mistake of
> > > doing a dist-upgrade on my testing VM master image and it pulled in a
> > > version of systemd (257.4-3) that has a broken hibernate.  Since I
> > > upgraded in place I don't have the old image so I'm spending my time
> > > currently debugging systemd ... normal service will hopefully resume
> > > shortly.
> > 
> > I found the systemd bug
> > 
> > https://github.com/systemd/systemd/issues/36888
> > 
> > And hacked around it, so I can confirm a simple hibernate/resume works
> > provided the sd_start_write() patches are applied (and the hooks are
> > plumbed in to pm).
> > 
> > There is an oddity: the systemd-journald process that would usually
> > hang hibernate in D wait goes into R but seems to be hung and can't be
> > killed by the watchdog even with a -9.  It's stack trace says it's
> > still stuck in sb_start_write:
> > 
> > [<0>] percpu_rwsem_wait.constprop.10+0xd1/0x140
> > [<0>] ext4_page_mkwrite+0x3c1/0x560 [ext4]
> > [<0>] do_page_mkwrite+0x38/0xa0
> > [<0>] do_wp_page+0xd5/0xba0
> > [<0>] __handle_mm_fault+0xa29/0xca0
> > [<0>] handle_mm_fault+0x16a/0x2d0
> > [<0>] do_user_addr_fault+0x3ab/0x810
> > [<0>] exc_page_fault+0x68/0x150
> > [<0>] asm_exc_page_fault+0x22/0x30
> > 
> > So I think there's something funny going on in thaw.
> 
> As Christian wrote, it seems systemd-journald does a memory store to
> mmapped file and gets blocked on sb_start_write() while doing the page
> fault. What's strange is that R state. Is the task really executing on some
> CPU or it only has 'R' state (i.e., got woken but never scheduled)?

I think the issue is that we need to also make pagefault based writers
such as systemd-journald freezable:

I don't think that's it. I think you're missing making pagefault writers
such
as systemd-journald freezable:

I don't think that's it. I think you're missing making pagefault writers such
as systemd-journald freezable:

diff --git a/include/linux/fs.h b/include/linux/fs.h
index b379a46b5576..528e73f192ac 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1782,7 +1782,8 @@ 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);
+                                  (level == SB_FREEZE_WRITE ||
+                                   level == SB_FREEZE_PAGEFAULT));
 }