Message-ID: <20080106204925.GA19601@redhat.com>
Date:	Sun, 6 Jan 2008 15:49:25 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Huang, Ying" <ying.huang@...el.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-pm@...ts.linux-foundation.org,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3 -mm] kexec jump -v8

On Thu, Jan 03, 2008 at 04:42:26PM +0800, Huang, Ying wrote:
> On Mon, 2007-12-31 at 14:26 -0500, Vivek Goyal wrote:
> [...]
> > Ok. But If I copy the /proc/vmcore to disk. Then I reboot the system 
> > and boot back into a kernel which is supposed to resume the hibernated
> > image. This kernel will not have any command line option jump_back_entry.
> > I  need to resume using krestore tool. How will a krestore tool decide that
> > image one wants to restore is a core file or a hibernated image?
> > 
> > In this context I thought an PT_NOTE might be useful.
> 
> In current implementation, when saving the memory image of hibernated
> kernel in hibernating kernel, the jump back entry from kernel command
> line should be saved at the same time, or the jump back entry from
> kernel command line can be saved as entry point of ELF hibernated image
> file by a modified makedumpfile. That is, 
> 
> - method 1:
> 
> jbe=`cat /proc/cmdline | tr -d ' ' '\n' | grep kexec_jump_back_entry | cut -d '=' -f 2`
> cp /proc/vmcore .
> echo $jbe > kexec_jump_back_entry
> 
> - method 2:
> 
> jbe=`cat /proc/cmdline | tr -d ' ' '\n' | grep kexec_jump_back_entry | cut -d '=' -f 2`
> makedumpfile <other options> -j $jbe /proc/vmcore <image file>
> 
> 
> Method 2 is better, because "jump back entry" is the very entry point of
> the hibernated image. The vmcore implementation can be changed to set
> entry point of /proc/vmcore if "kexec_jump_back_entry" kernel command
> line option is present. I did not add this to kernel because it can be
> done by user space tool "makedumpfile", and one of the guideline is to
> keep kernel part as simple as possible.
> 

I think storing the jump back entry in ELF header should be better.

> With method 2, the ordinary "core file" and "hibernated image" can be
> distinguished by the "entry point".
> 

Hmm... That's one possible way of differentiating between two kind of
files.

> 
> The "jump back entry" can be saved with PT_NOTE too. And more
> information can be saved with PT_NOTE. I have ever defined another
> mechanism to pass information between two kernels too. I will describe
> it later in this mail.
> 

Actually I meant to store a string in PT_NOTE, something like "RESUMABLE"
or "HIBERNATED" etc to differentiate between a normal core file and a core
file which can be resumed. Entry point should be stored in ELF header
only.


> > > As for ELF NOTE, in fact, the ELF NOTE does not work for resumable
> > > kernel. Because the contents of source page and destination page is
> > > swapped during kexec, and the kernel access the destination page
> > > directly during parsing ELF NOTE. All memory that is swapped need to be
> > > accessed via the backup pages map (image->head). I think these
> > > information can be exchanged between two kernels via kernel command line
> > > or /proc/kimgcore.
> > > 
> > 
> > Why should we exchange backup page map information between two kernels?
> > What data has been swapped and how to restore it back should be known
> > to the kernel who did it. I think the memory info and address of ELF
> > headers we can still pass to second kernel the same way we do for
> > /proc/vmcore. The only difference is that purgatory shall have to modify 
> > the command line to reflect the new address of ELF headers just before
> > jumping to new kernel (because of swapping).
> > 
> > We can also modify the actual ELF headers in purgatory to reflect the
> > right data (because of swapping) and new kernel will not have to know
> > anything about swapping.
> > 
> > So I thought of following sequence.
> > 
> > Lets say there is production kernel A and there is helper kernel B (B
> > will save the hibernated image of A).
> > 
> > - Boot into A.
> > - Load kernel B using --load-preserve-context.
> > - This load operation can also create the Elf headers.   
> > - Kexec -e will start hibernation of A. Pages shall be swapped. Control
> >   will be transferred to purgatory.
> > - Purgatory will readjust the ELF headers and command line based on 
> >   the swapping done and jump to new kernel.
> > - New kernel will retrieve the elf headers and export the memory of
> >   hibernated kernel through /proc/vmcore.
> > 
> > Hence there is no need to communicate swapped pages map between two
> > kernels.
> 
> One original page will map to one backup page (the contents of original
> page and backup page will be swapped). It is possible that the entries
> number of map equals the number of backup pages.
> 
> If the helper kernel B uses 16M memory, there will be 4096 backup pages.
> One PT_LOAD header is needed for each backup page to record the map
> between the original page to the backup page. So, the memory needed for
> PT_LOAD headers will be 32 * 4k = 128k.
> 
> If the size of PT_LOAD headers and the "readability" of headers
> information of /proc/vmcore is not a big issue, I think this is a good
> idea.
> 

Ok. So if backup page allocation is not contiguous then we will end
up with one PT_LOAD header per backup page, hence large number of PT_LOAD
headers. I know it is little ugly, but still think that it is a better
way.   

> In addition to revising the ELF headers in purgatory, the /proc/kimgcore
> interface can also be used to revise the ELF headers in kernel A. But
> this needs to export backup pages map information to user space (another
> sysfs/procfs file?).

This is kind of two step load process. Load image, then export backup
page info to user space and then readjust the headers through
/proc/kimgcore. I think purgatory doing it is cleaner.
 
> 
> [...]
> > > The image of B is made as you said. And it can be restored as
> > follow:
> > > 
> > > /sbin/kexec -l --args-none --flags=0x2 <kimgecore>
> > > /sbin/kexec -e
> > > 
> > > That is, the image of B is loaded as a ordinary ELF file. A option
> > > to /sbin/kexec named --flags are added to specify the
> > > KEXEC_PRESERVE_CONTEXT flags for sys_kexec_load. This has been tested.
> > 
> > Shouldn't we be able to load this image using --load-preseve-context? Why
> > to create another otion --flags.
> 
> Yes. This seems better. I will change it.
> 
> > So there seems to be two different kind of load options for resuming
> > for two different kind of images.
> > 
> > - For /proc/vmcore kind of images use krestore (which will use
> >   --load-jump-back-helper)
> > 
> > - For /proc/kimgcore kind of images use normal kexec -l.
> > 
> > This is little confusing. How will one differentiate between two kind
> > of files? In fact the semantics between two kind of files are not very
> > clear. We need to do some kind of unification here and make the whole
> > thing somewhat simple.
> 
> I think there may be no difference between two kinds of files. Nothing
> prevents hibernated image produced through /proc/vmcore loaded with
> normal kexec -l. I have just tested it. This means we need not use
> krestore to resume the hibernated kernel after enhancing /sbin/kexec
> (because it uses too much anonymous memory). And, the resuming process
> will be more smooth.
> 

Ok. So in a nutshell, any resumable image (be it is generated through
/proc/vmcore or /proc/kimgcore) can be launched in the same way  (I think
kexec --load-preserve-context). ?

In fact, if we are modifying the ELF headers in purgatory to reflect the
swapped page action, then we don't require /proc/kimgcore interface
at all?


> For completion (although may be not a big issue now):
> 
> Whether using "krestore" or normal "kexec -l" depends on the memory used
> by current kernel (/proc/iomem) and memory in image file (PT_LOAD
> headers). If all memory in image file is outside memory used by current
> kernel, "krestore" should be used. If all memory in image file is inside
> memory used by current kernel, normal "kexec -l" should be used. If
> there is intersection set between memory in image file and memory of
> current kernel, the image file can not be loaded.
> 

I think kexec should mask that difference. A user should be able to load
a resumable image either by using "kexec -l" or "kexec
--load-preserve-context" depending on whether user wants to come back to
orignal kernel in future or not. Kexec-tools should recognize the image
as resumable. Any page outside the current kernel can be written to
final location through /dev/oldmem. And any pages which overlap with the
current kernel, should be moved to destination when actual kexec 
happens (existing functionality).

This will require merging krestore and kexec user space functionality
so that resuming a hibernated image is effectively a "Kexec -l"
operation.

So a user will not worry about whether he is kexecing a fresh kernel
(bzImage or vmlinux) or resuming a already booted kernel. Kexec tools
should determine that and setup the entry point accordingly (might
require some purgatory changes to take care of transition while jumping
to resume hibernated image).


> > > 
> > > An example:
> > > 
> > > 1. kernel A kexec kernel B, provides vmcoreinfo in kernel command line.
> > 
> > I think vmcoreinfo is stored in a PT_NOTE and passed to second kernel
> > parses it while parsing all ELF headers? No separate ptr or anything
> > else is passed to second kernel on command line.
> 
> In current implementation, the PT_NOTE does not work, so a kernel
> command line can be used to pass the information. And it can be changed
> for each resuming. easier than changing PT_NOTE.
> 
> > > 2. kernel B jump back to kernel A
> > > 3. in kernel A, the memory image of kernel B is saved via /proc/kimgcore
> > > 4. system reboot, kernel C is booted
> > > 5. kernel C load the memory image of kernel B, and jump back to kernel B
> > > 
> > > In step 5, the vmcoreinfo provided in step 1 is invalid, so a
> > > communication method is needed to tell kernel B the new one. This can be
> > > done via amending /proc/kimgcore. For example, change the memory of the
> > > kernel command line of kernel B.
> > 
> > I think modifying a kernel's data structures externally without kernel
> > knowing it is dangerous. 
> 
> Yes. It is dangerous. This is not a good example. But, I think it is a
> useful feature. For example, the "setup page" you proposed later can be
> setup via amending /proc/kimgcore.
> 
> And, it is more useful for "invoking some code in physical mode". The
> procedure is something as follow:
> 
> 1. load some code executing in physical mode via kexec --load-preserve-context.
> 2. setup the parameters via amending /proc/kimgcore
> 3. execute the code in physical mode via kexec -e
> 4. get the result via reading /proc/kimgcore
> 5. setup another groups of parameters via amending /proc/kimgcore
> ...

This seems to be extended functionlity. If your focus is "Kexec based
hibernation" then I would think of initially keeping the implementation
simple and keeping patches small. Make kexec based hibernation work
and then extend functionality for other purposes.


> 
> > I would think that initially we can keep it simple and not suppot
> > resuming. Resuming is little tough as you need to make sure kernel's
> > complete env has been restored back. But in this case, we are playing
> > with resume env, for example, changing of vmcoreinfo etc.
> > 
> > Capability to resume the already booted kernel (Kernel B here), creates
> > a coupling with the kernel it has booted with in the past (Kernel A) and
> > that's why you end up modifying various things once you swith to B 
> > from C. I think we should boot B fresh all the time to save the hibernated
> > image of kernel A or C (Though it will take more time for hibernation).
> > It keeps things simple.
> > 
> > If you still want to support resuming the kernel, then I would think that in
> > resume path kernel should re-initialize/re-construct /proc/vmcore
> > (parse vmcoreinfo again) etc, instead of modifying the image of loaded kernel
> > through /proc/kimgcore interface.
> > 
> > We can probably think of a setup page where all the needed data is put and
> > this page is used as a communication medium between two kernels (Something
> > like the way, a page of data is passed between boot-loader and kernel to
> > communicate the info like command line etc).
> > 
> > So in the above example, kernel C will pass some data to kernel B (on
> > setup page). This data will also include the pointer to new ELF headers
> > (which also includes vmcoreinfo PT_NOTE). Resume path of kernel B
> > will parse this data and re-initialize vmcore accordingly.
> 
> Yes. A communication mechanism is needed between two kernels. The "setup
> page" and "PT_NOTE" earlier you proposed are two such mechanisms. It
> seems that the "setup page" is more powerful. I have defined a mechanism
> like "setup page" in my previous version. It is as follow:
> 
> 1. The second half of "control page" is used as the "setup page". That
> is, jump_back_entry + 0x800.
> 2. A magic number is provided in "setup page" to let krestore to
> distinguish between "normal executable" and "hibernated iamge" by check
> the magic number.
> 3. Other information such as vmcoreinfo can be added to "setup page".
> 4. Before one kernel jump to another kernel, the parameters are prepared
> by current kernel.
> 5. One kernel can check the parameters of another kernel by
> reading /proc/vmcore or /proc/kimgcore.
> 6. When memory image is saved in file. The parameters of hibernated
> kernel can be check by reading memory location jump_back_entry + 0x800.
> 
> You can check the details of this mechanism in my previous patch with
> title:
> 
> [PATCH 1/4 -mm] kexec based hibernation -v7 : kexec jump
> 
> The main issue of this mechanism is that: it is a kernel-to-kernel
> communication mechanism, while Eric Biederman thinks we should use only
> user-to-user communication mechanism. And he is not persuaded now.
> 
> Because kernel operations such as re-initialize/re-construct
> the /proc/vmcore, etc are needed for kexec jump or resuming. I think a
> "kernel-to-kernel" mechanism may be needed. But I don't know if Eric
> Biederman will agree with this.

Hmm... Personally I am more inclined to exchanging information between
two kernels on setup page, in a standard format (using ELF headers etc).
This information can be prepared by kexec-tools in user space and be). 
modified by purgatory (during transition to reflect the swapped pages.
Alternatively, one can modify this setup page info from user space through
some /proc/kimgcore like interface.  I prefer the first one...

Eric, what do you think abou the whole thing?

Thanks
Vivek

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