Re: [RFC PATCH 2/2] swsusp: make it possible to hibernate to device mapper devices

Mikulas Patocka <[email protected]>:
https://lore.kernel.org/linux-pm/[email protected]/ :
> Askar Safin requires swap and hibernation on the dm-integrity device mapper
> target because he needs to protect his data.
> 
> This hits two problems:
> 1. The kernel doesn't send the flush bio to the hibernation device after
>    writing the image and before powering off - this is easy to fix
> 2. The dm-integrity target keeps parts of the device in-memory - it keeps
>    a journal and a dm-bufio cache in memory. If we hibernate and resume,
>    the content of memory no longer matches the data on the hibernate
>    partition and that may cause spurious errors - this is hard to fix

Let me add some more info on this patchset.

First of all, I already solved the problem for me personally:
I wrote hackish patch, which fixes the problem. My patch is tested on
my real hardware under load. I successfully use it for 2 weeks
(I hibernated a lot of times during this period.)
The patch is absolutely rock solid, and I absolutely sure it is correct.
Unfortunately, it is not generic, it is tied to my particular configuration,
it hard codes paths (!!!), and hence is non-upstreamable.

Here is this patch for your information:
https://zerobin.net/?ad6142bd67df015a#68Az6yBUxHA3AXB7jY1+clSRnR745olFHAByxwPGM08= .

Feel free to use code from it.

So I personally is not in hurry, I already have solution, which works for me.
(But I am still available for testing.)


Your patch has a problem: after "notify_swap_device" call, the pages can
still be swapped out. "pm_restrict_gfp_mask" call in "hibernation_snapshot"
prevents further swapping. Thus "notify_swap_device" should be called
after "pm_restrict_gfp_mask" (but read on).

I attempted to create test case, which would expose this problem. And I was
unable to do so. Still I believe this is a real problem.


Also, our problem is very similar to reason of introducing "filesystems_freeze"
( https://elixir.bootlin.com/linux/v6.18-rc7/source/kernel/power/hibernate.c#L824 ).

See problem description here:
https://lore.kernel.org/all/0a76e074ef262ca857c61175dd3d0dc06b67ec42.camel@HansenPartnership.com/ .

See also https://lwn.net/Articles/1018341/ .

(See also this huge thread
https://lore.kernel.org/all/[email protected]/ .)

"filesystems_freeze" logic is already implemented in mainline. It is gated
behind /sys/power/freeze_filesystems .

As you can see, authors of "filesystems_freeze" attempted to solve similar
problem. Thus, we should probably flush buffers in "filesystems_freeze" call.
Ideally, flushing of dm-integrity should be correctly ordered with freezing of filesystems
to support complex storage hierarchies (i. e. swap on dm-integrity on loop
device on some filesystem, etc).

But... call to "filesystems_freeze" happens before "pm_restrict_gfp_mask" call.

So... in this point I gave up, and I don't know what to do (i. e. what the upstream
kernel should do).


Feel free to ask any questions.


I changed CCs for further exposure.

-- 
Askar Safin