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From: Baoquan He <bhe@redhat.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, robh+dt@kernel.org,
	dan.j.williams@intel.com, nicolas.pitre@linaro.org,
	josh@joshtriplett.org, fengguang.wu@intel.com, bp@suse.de,
	andy.shevchenko@gmail.com, patrik.r.jakobsson@gmail.com,
	airlied@linux.ie, kys@microsoft.com, haiyangz@microsoft.com,
	sthemmin@microsoft.com, dmitry.torokhov@gmail.com,
	frowand.list@gmail.com, keith.busch@intel.com,
	jonathan.derrick@intel.com, lorenzo.pieralisi@arm.com,
	bhelgaas@google.com, tglx@linutronix.de, brijesh.singh@amd.com,
	jglisse@redhat.com, thomas.lendacky@amd.com,
	gregkh@linuxfoundation.org, baiyaowei@cmss.chinamobile.com,
	richard.weiyang@gmail.com, devel@linuxdriverproject.org,
	linux-input@vger.kernel.org, linux-nvdimm@lists.01.org,
	devicetree@vger.kernel.org, linux-pci@vger.kernel.org,
	ebiederm@xmission.com, vgoyal@redhat.com, dyoung@redhat.com,
	yinghai@kernel.org, monstr@monstr.eu, davem@davemloft.net,
	chris@zankel.net, jcmvbkbc@gmail.com, gustavo@padovan.org,
	maarten.lankhorst@linux.intel.com, seanpaul@chromium.org,
	linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	kexec@lists.infradead.org
Subject: Re: [PATCH v7 4/4] kexec_file: Load kernel at top of system RAM if required
Date: Wed, 25 Jul 2018 10:21:15 +0800
Message-ID: <20180725022115.GH6480@MiWiFi-R3L-srv> (raw)
In-Reply-To: <20180719124444.c893712cca2e6f2649d1bc0d@linux-foundation.org>

Hi Andrew,

On 07/19/18 at 12:44pm, Andrew Morton wrote:
> On Thu, 19 Jul 2018 23:17:53 +0800 Baoquan He <bhe@redhat.com> wrote:
> > > As far as I can tell, the above is the whole reason for the patchset,
> > > yes?  To avoid confusing users.
> > 
> > 
> > In fact, it's not just trying to avoid confusing users. Kexec loading
> > and kexec_file loading are just do the same thing in essence. Just we
> > need do kernel image verification on uefi system, have to port kexec
> > loading code to kernel. 
> > 
> > Kexec has been a formal feature in our distro, and customers owning
> > those kind of very large machine can make use of this feature to speed
> > up the reboot process. On uefi machine, the kexec_file loading will
> > search place to put kernel under 4G from top to down. As we know, the
> > 1st 4G space is DMA32 ZONE, dma, pci mmcfg, bios etc all try to consume
> > it. It may have possibility to not be able to find a usable space for
> > kernel/initrd. From the top down of the whole memory space, we don't
> > have this worry. 
> > 
> > And at the first post, I just posted below with AKASHI's
> > walk_system_ram_res_rev() version. Later you suggested to use
> > list_head to link child sibling of resource, see what the code change
> > looks like.
> > http://lkml.kernel.org/r/20180322033722.9279-1-bhe@redhat.com
> > 
> > Then I posted v2
> > http://lkml.kernel.org/r/20180408024724.16812-1-bhe@redhat.com
> > Rob Herring mentioned that other components which has this tree struct
> > have planned to do the same thing, replacing the singly linked list with
> > list_head to link resource child sibling. Just quote Rob's words as
> > below. I think this could be another reason.
> > 
> > ~~~~~ From Rob
> > The DT struct device_node also has the same tree structure with
> > parent, child, sibling pointers and converting to list_head had been
> > on the todo list for a while. ACPI also has some tree walking
> > functions (drivers/acpi/acpica/pstree.c). Perhaps there should be a
> > common tree struct and helpers defined either on top of list_head or a
> > ~~~~~
> > new struct if that saves some size.
> 
> Please let's get all this into the changelogs?

Sorry for late reply because of some urgent customer hotplug issues.

I am rewriting all change logs, and cover letter. Then found I was wrong
about the 2nd reason. The current kexec_file_load calls
kexec_locate_mem_hole() to go through all system RAM region, if one
region is larger than the size of kernel or initrd, it will search a
position in that region from top to down. Since kexec will jump to 2nd
kernel and don't need to care the 1st kernel's data, we can always find
a usable space to load kexec kernel/initrd under 4G.

So the only reason for this patch is keeping consistent with kexec_load
and avoid confusion.

And since x86 5-level paging mode has been added, we have another issue
for top-down searching in the whole system RAM. That is we support
dynamic 4-level to 5-level changing. Namely a kernel compiled with
5-level support, we can add 'no5lvl' to force 4-level. Then jumping from
a 5-level kernel to 4-level kernel, e.g we load kernel at the top of
system RAM in 5-level paging mode which might be bigger than 64TB, then
try to jump to 4-level kernel with the upper limit of 64TB. For this
case, we need add limit for kexec kernel loading if in 5-level kernel.

All this mess makes me hesitate to choose a deligate method. Maybe I
should drop this patchset.

> 
> > > 
> > > Is that sufficient?  Can we instead simplify their lives by providing
> > > better documentation or informative printks or better Kconfig text,
> > > etc?
> > > 
> > > And who *are* the people who are performing this configuration?  Random
> > > system administrators?  Linux distro engineers?  If the latter then
> > > they presumably aren't easily confused!
> > 
> > Kexec was invented for kernel developer to speed up their kernel
> > rebooting. Now high end sever admin, kernel developer and QE are also
> > keen to use it to reboot large box for faster feature testing, bug
> > debugging. Kernel dev could know this well, about kernel loading
> > position, admin or QE might not be aware of it very well. 
> > 
> > > 
> > > In other words, I'm trying to understand how much benefit this patchset
> > > will provide to our users as a whole.
> > 
> > Understood. The list_head replacing patch truly involes too many code
> > changes, it's risky. I am willing to try any idea from reviewers, won't
> > persuit they have to be accepted finally. If don't have a try, we don't
> > know what it looks like, and what impact it may have. I am fine to take
> > AKASHI's simple version of walk_system_ram_res_rev() to lower risk, even
> > though it could be a little bit low efficient.
> 
> The larger patch produces a better result.  We can handle it ;)

For this issue, if we stop changing the kexec top down searching code,
I am not sure if we should post this replacing with list_head patches
separately.

Thanks
Baoquan
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  reply	other threads:[~2018-07-25  2:21 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-07-18  2:49 [PATCH v7 0/4] resource: Use list_head to link sibling resource Baoquan He
2018-07-18  2:49 ` [PATCH v7 1/4] resource: Move reparent_resources() to kernel/resource.c and make it public Baoquan He
2018-07-18  2:49 ` [PATCH v7 2/4] resource: Use list_head to link sibling resource Baoquan He
2018-07-18  2:49 ` [PATCH v7 3/4] resource: add walk_system_ram_res_rev() Baoquan He
2018-07-18  2:49 ` [PATCH v7 4/4] kexec_file: Load kernel at top of system RAM if required Baoquan He
2018-07-18 22:33   ` Andrew Morton
2018-07-19 15:17     ` Baoquan He
2018-07-19 19:44       ` Andrew Morton
2018-07-25  2:21         ` Baoquan He [this message]
2018-07-23 14:34       ` Michal Hocko
2018-07-25  6:48         ` Baoquan He
2018-07-26 12:59           ` Michal Hocko
2018-07-26 13:09             ` Baoquan He
2018-07-26 13:12               ` Michal Hocko
2018-07-26 13:14                 ` Michal Hocko
2018-07-26 13:37                   ` Baoquan He
2018-07-26 14:01                     ` Michal Hocko
2018-07-26 15:10                       ` Baoquan He

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