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

[Jan Kara]

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@xxxxxxxxxx
> > 
> > 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)?

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR