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Message-ID: <20180725064813.GI6480@MiWiFi-R3L-srv>
Date:   Wed, 25 Jul 2018 14:48:13 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, robh+dt@...nel.org,
        dan.j.williams@...el.com, nicolas.pitre@...aro.org,
        josh@...htriplett.org, fengguang.wu@...el.com, bp@...e.de,
        andy.shevchenko@...il.com, patrik.r.jakobsson@...il.com,
        airlied@...ux.ie, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, dmitry.torokhov@...il.com,
        frowand.list@...il.com, keith.busch@...el.com,
        jonathan.derrick@...el.com, lorenzo.pieralisi@....com,
        bhelgaas@...gle.com, tglx@...utronix.de, brijesh.singh@....com,
        jglisse@...hat.com, thomas.lendacky@....com,
        gregkh@...uxfoundation.org, baiyaowei@...s.chinamobile.com,
        richard.weiyang@...il.com, devel@...uxdriverproject.org,
        linux-input@...r.kernel.org, linux-nvdimm@...ts.01.org,
        devicetree@...r.kernel.org, linux-pci@...r.kernel.org,
        ebiederm@...ssion.com, vgoyal@...hat.com, dyoung@...hat.com,
        yinghai@...nel.org, monstr@...str.eu, davem@...emloft.net,
        chris@...kel.net, jcmvbkbc@...il.com, gustavo@...ovan.org,
        maarten.lankhorst@...ux.intel.com, seanpaul@...omium.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kexec@...ts.infradead.org
Subject: Re: [PATCH v7 4/4] kexec_file: Load kernel at top of system RAM if
 required

On 07/23/18 at 04:34pm, Michal Hocko wrote:
> On Thu 19-07-18 23:17:53, Baoquan He wrote:
> > 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. 
> 
> I do not have the full context here but let me note that you should be
> careful when doing top-down reservation because you can easily get into
> hotplugable memory and break the hotremove usecase. We even warn when
> this is done. See memblock_find_in_range_node

Kexec read kernel/initrd file into buffer, just search usable positions
for them to do the later copying. You can see below struct kexec_segment, 
for the old kexec_load, kernel/initrd are read into user space buffer,
the @buf stores the user space buffer address, @mem stores the position
where kernel/initrd will be put. In kernel, it calls
kimage_load_normal_segment() to copy user space buffer to intermediate
pages which are allocated with flag GFP_KERNEL. These intermediate pages
are recorded as entries, later when user execute "kexec -e" to trigger
kexec jumping, it will do the final copying from the intermediate pages
to the real destination pages which @mem pointed. Because we can't touch
the existed data in 1st kernel when do kexec kernel loading. With my
understanding, GFP_KERNEL will make those intermediate pages be
allocated inside immovable area, it won't impact hotplugging. But the
@mem we searched in the whole system RAM might be lost along with
hotplug. Hence we need do kexec kernel again when hotplug event is
detected.

#define KEXEC_CONTROL_MEMORY_GFP (GFP_KERNEL | __GFP_NORETRY)


struct kexec_segment {
        /*
         * This pointer can point to user memory if kexec_load() system
         * call is used or will point to kernel memory if
         * kexec_file_load() system call is used.
         *
         * Use ->buf when expecting to deal with user memory and use ->kbuf
         * when expecting to deal with kernel memory.
         */
        union {
                void __user *buf;
                void *kbuf;
        };
        size_t bufsz;                                                                                                                             
        unsigned long mem;
        size_t memsz;
};

Thanks
Baoquan

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