linux-kernel - Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system

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Date: Mon, 24 Jun 2024 07:43:47 -0500
From: John Groves <John@...ves.net>
To: Miklos Szeredi <miklos@...redi.hu>
Cc: linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev
Subject: Re: [RFC PATCH 00/20] Introduce the famfs shared-memory file system

On 24/05/23 03:57PM, Miklos Szeredi wrote:
> [trimming CC list]
> 
> On Thu, 23 May 2024 at 04:49, John Groves <John@...ves.net> wrote:
> 
> > - memmap=<size>!<hpa_offset> will reserve a pretend pmem device at <hpa_offset>
> > - memmap=<size>$<hpa_offset> will reserve a pretend dax device at <hpa_offset>
> 
> Doesn't get me a /dev/dax or /dev/pmem
> 
> Complete qemu command line:
> 
> qemu-kvm -s -serial none -parallel none -kernel
> /home/mszeredi/git/linux/arch/x86/boot/bzImage -drive
> format=raw,file=/home/mszeredi/root_fs,index=0,if=virtio -drive
> format=raw,file=/home/mszeredi/images/ubd1,index=1,if=virtio -chardev
> stdio,id=virtiocon0,signal=off -device virtio-serial -device
> virtconsole,chardev=virtiocon0 -cpu host -m 8G -net user -net
> nic,model=virtio -fsdev local,security_model=none,id=fsdev0,path=/home
> -device virtio-9p-pci,fsdev=fsdev0,mount_tag=hostshare -device
> virtio-rng-pci -smp 4 -append 'root=/dev/vda console=hvc0
> memmap=4G$4G'
> 
> root@kvm:~/famfs# scripts/chk_efi.sh
> This system is neither Ubuntu nor Fedora. It is identified as debian.
> /sys/firmware/efi not found; probably not efi
>  not found; probably nof efi
> /boot/efi/EFI not found; probably not efi
> /boot/efi/EFI/BOOT not found; probably not efi
> /boot/efi/EFI/ not found; probably not efi
> /boot/efi/EFI//grub.cfg not found; probably nof efi
> Probably not efi; errs=6
> 
> Thanks,
> Miklos

I'm baffled as to why the memmap thing is not working for you. I don't see
anything amiss in your config file, but the actual plumbing of that kernel 
option isn't anything I've worked on. Out of curiosity, are you running on x86?

Have you tried the 's/$/!/' method with memmap? That should give you a pmem
device instead, which you will see with 'ndctl list', and can convert to
devdax with ndctl (recipe above in this thread). Note that 4GiB is the minimum
size that famfs supports.

A quick status on where I am with famfs: I've made progress on my substantial
learning curve with fuse, and have come up with a strategy for the famfs fuse 
daemon to access metadata in a way that leverages the current famfs user space 
without excessive re-writing (which is encouraging). 

I haven't started test-hacking dax_iomap_* enabled files into the fuse
kmod yet; initial RFCs in that area are probably a few weeks out, but 
definitely coming - undoubtedly with a lot of questions.

Regards,
John