linux-kernel - Re: [PATCH v7 4/4] kexec_file: Load kernel at top of system RAM if required

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Date:   Mon, 23 Jul 2018 16:34:43 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
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,
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        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 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
-- 
Michal Hocko
SUSE Labs