Re: btrfs crash on armv7

[riteshh <riteshh@linux.ibm.com>]

Please excuse my silly queries here.

On 21/04/08 04:38PM, Qu Wenruo wrote:
>
>
> On 2021/4/8 下午4:16, Joe Hermaszewski wrote:
> > It took a while but I managed to get hold of another one of these
> > arm32 boards. Very disappointingly this exact "bitflip" is still
> > present (log enclosed).
>
> Yeah, we got to the conclusion it's not bitflip, but completely 32bit
> limit on armv7.
>
> For ARMv7, it's a 32bit system, where unsigned long is only 32bit.
>
> This means, things like page->index is only 32bit long, and for 4K page
> size, it also means all filesystems (not only btrfs) can only utilize at
> most 16T bytes.
>
> But there is pitfall for btrfs, btrfs uses its internal address space
> for its meatadata, and the address space is U64.

Can you pls point me to the code you are referring here?
So IIUC, you mean since page->index can hold a value which can be upto 32bit in
size so the maximum FS address range which can be accessed is 16T.
This should be true in general for any FS no?

>
> And furthermore, for btrfs it can have metadata at bytenr way larger
> than the total device size.

Is this because of multi-device support?

> This is possible because btrfs maps part of its address space to real
> disks, thus it can have bytenr way larger than device size.

Please a code pointing to that will help me understand this better.
Thanks.

>
> But this brings to a problem, 32bit Linux can only handle 16T, but in
> your case, some of your metadata is already beyond 16T in btrfs address
> space.

Sorry I am not much aware of the history. Was this disk mkfs on 64-bit system
and then connected to a 32bit board?

This also brings me to check with you about other filesystems.
See the capacity section from below wiki[1]. Depending upon the host OS
limitation on the max size of the filesystem may vary right?

[1] https://en.wikipedia.org/wiki/XFS

-ritesh

>
> Then a lot of things are going to be wrong.
>
> I have submitted a patch to do extra check, at least allowing user to
> know this is the limit of 32bit:
> https://patchwork.kernel.org/project/linux-btrfs/patch/20210225011814.24009-1-wqu@suse.com/
>
> Unfortunately, this will not help existing fs though.
>