running into OOM killer with kexec loading large ramdisk

[Tobias Powalowski <tobias.powalowski@googlemail.com>]

Hi, thanks for the explanation,
I got it now working, with running the kexec call in the background
and deleting the initramfs in the foreground after sleep 1.
I still think kexec should handle this with less memory needed.
greetings
tpowa

Am Fr., 25. M?rz 2022 um 13:26 Uhr schrieb Philipp Rudo <prudo@redhat.com>:
>
> Hi Tobias,
>
> On Wed, 23 Mar 2022 12:14:59 +0100
> Tobias Powalowski <tobias.powalowski@googlemail.com> wrote:
>
> > Hi,
> > again me,
> > I try to load a 900MB ramdisk on a ramfs rootfs  with kexec and a 10MB kernel.
> > With 2800MB RAM assigned to qemu.
> > Memory free by /proc/memstat: 2.2GB
> > It keeps on OOM killed while executing:
> > kexec -l kernel --initrd=initrd.img
> > I can safely unpack the initrd in the ramfs without getting OOM killed.
> > What is kexec doing wrong here?
>
> I don't think that kexec is doing anything wrong here.
>
> The kexec_load syscall is designed in a way that user space prepares
> everything in a huge buffer and passes a pointer to it to the system
> call. The systemcall then needs to copy everything from the user buffer
> to a kernel buffer. So there are three copies of the initrd in memory
> (including the one in ramfs). Together they take up 3 * 900MB = 2.7GB.
> So basically all the memory of your guest.
>
> You can try using the kexec_file_load systemcall. That at least
> eliminates the copy in the user buffer so there are only two copies
> left. 400MB of free memory is still quite scarce, though.
>
> Thanks
> Philipp
>


-- 
Tobias Powalowski
Arch Linux Developer & Package Maintainer (tpowa)
https://www.archlinux.org
tpowa at archlinux.org

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