Hi Tao,
On Wed, 6 Apr 2022 18:55:38 +0800
Tao Liu <ltao(a)redhat.com> wrote:
On Tue, Apr 5, 2022 at 5:37 PM Philipp Rudo <prudo(a)redhat.com>
wrote:
>
> Hi Coiby,
> Hi Tao,
>
> On Sat, 2 Apr 2022 16:43:37 +0800
> Coiby Xu <coxu(a)redhat.com> wrote:
>
> > Hi Tao,
> >
> > On Sat, Apr 02, 2022 at 02:14:38PM +0800, Tao Liu wrote:
> > >Hi Philipp,
> > >
> > >On Wed, Mar 30, 2022 at 11:34 PM Philipp Rudo <prudo(a)redhat.com>
wrote:
> > [...]
> > >
> > >> > implemented the reservation in the kernel side, as part of the
kernel
> > >> > crashkernel=auto implementation.
> >
> >
> > For swiotlb, I think Dave and Emma prefer to do it in kernel space.
> > Maybe you can try a kernel space solution first like [1]?
> >
> > [1]
https://lore.kernel.org/lkml/20190910151341.14986-3-kasong@redhat.com/
> >
> > >> >
> > >> > For rhel9 however, crashkernel=auto has been mirrored to
userspace
> > >> > kexec-tools, so this patch will enable the swiotlb reservation
for AMD
> > >> > sme/sev in kexec-tools as well.
> > >>
> > >> Not really about this patch but I find it weird that we adjust the
> > >> crashkernel=auto value for swiotlb but not for other cases wher we
know
> > >> that we need a larger crashkernel, e.g. when the dump target is on a
> > >> LUKS encrypted device. I understand that in the past, when
> > >> crashkernel=auto was handled by the kernel, we simply didn't know
about
> > >> what the dump target will be when we allocated the crashkernel during
> > >> boot. But today, when crashkernel=auto is handled in userspace we
can.
> > >>
> > >
> > >I didn't have much context on LUKS, by looking into the code, I see
only
> > >"kdumpctl estimate" has the LUKS related crashkernel value
increment. It needs
> > >the user to adjust crashkernel value manually by estimate-and-set. Maybe
we
> > >can merge the crashkernel adjustment of swiotlb and luks together,
> > >so luks don't need to "kdumpctl estimate" manually anymore?
> >
> > For LUKS, a kernel-space solution by reusing LUKS master key is
> > preferred because a) LUKS requires too much memory b) we can't expect
> > the user to wait at the screen to input the passphrase to open the disk
> > when kernel crashes. Let's see how far my work [2] can go.
>
Yeah, after I played around with LUKS for a while, I agree with your point.
kernel side implementation is better for luks.
Just for curiosity, as for the case b), can't we use a password file instead
of inputting passphrase manually every time? Like:
$ cat /etc/fstab
/dev/mapper/mydata /mnt ext4 defaults 0 0
$ cat /etc/crypttab
mydata /dev/vdb /root/lukspass
$ cryptsetup luksAddKey /dev/vdb /root/lukspass
If /root/lukspass is accessible, luks device can be automatically
mounted, thus no
user interaction needed.
Sounds like an interesting idea. My only concern is that we introduce a
potential security risk, e.g. somebody might extract the key file from
the initrd and use it to access a disk without permission. But I don't
have much experience in the area. So I think it's definitely something
worth to further investigate.
> you are right that it is better to solve the problem with LUKS
in the
> kernel. I only used it as an example where today we know that we need
> more memory but don't automatically increase the crashkernel. So why do
> it for swiotlb but for others? What's the reason for this inconsistency?
By looking into the "kdumpctl estimate" code, the memory estimating for luks
is
calculated dynamically, depending on how many crypt devices there are, and
their "Memory:" field status.
However for amd sme/sev swiotlb, the reserved memory for 2nd kernel is 64M,
see rhel-8 kernel commit ab30d3f ("[x86] Reserve at most 64M of SWIOTLB
memory for crashkernel").
I implement it in the kexec-tools side, because the rhel-8 patch is
part of craskernel=auto
implementation. As far as I know crashkernel=auto has been mirrored to
kexec-tools side, so it is reasonable for me to make it in kexec-tools
side as well.
I fully understand. But remember that the crashkernel needs to be
reserved extremely early during boot. Even before the initramfs
is mounted. So when crashkernel=auto was implemented in the kernel we
simply didn't had information about many things, like what the root
device is and if it is encrypted. swiotlb was one of the few exceptions.
By moving crashkernel=auto to the kexec-tools we now have the
information and could use it. If we decide to do so (see Coibys mail).
>
> And a little bit more general. What is the relation between 'kdumpctl
> reset-crashkernel' and 'kdumctl estimate'? The way I see it the
> workflow we suggest to users currently is
>
> 1. $ kdumpctl reser-crashkernel
> 2. -> test if it works, if not
> 3. $ kdumpctl estimate
> 4. -> set crashkernel manually to suggested value
> 5. -> test if it works, if not
> 6. -> set crashkernel manually to larger value
> 7. -> test if it works, if not go back to 6
>
Thanks for the summarization. I happened to lose
the context, which confused me a lot when reading the code.
np
> I don't understand why we need the separate 'kdumpctl
estimate' step in
> between. In my opinion the following workflow would be much simpler
> from a user perspective
>
> 1. $ kdumpctl reset-crashkernel
> (calls kdumpctl estimate internally using current default values
> as minimum values)
> 2. -> test if it woks, if not
> 3. -> set crashkernel manually larger value
> 4. -> test if it works, if not go back to 3
>
Agreed!
Thanks
Philipp