Re: Since 6.10 - kernel oops/panics on G4 macmini due to change in drivers/ata/pata_macio.c

[Niklas Cassel <cassel@kernel.org>]

On Wed, Aug 14, 2024 at 10:20:55PM +1000, Michael Ellerman wrote:
> Niklas Cassel <cassel@kernel.org> writes:
> > On Tue, Aug 13, 2024 at 10:32:36PM +1000, Michael Ellerman wrote:
> >> Niklas Cassel <cassel@kernel.org> writes:
> >> > On Tue, Aug 13, 2024 at 07:49:34AM +0200, Jonáš Vidra wrote:
> ...
> >> >> ------------[ cut here ]------------
> >> >> kernel BUG at drivers/ata/pata_macio.c:544!
> >> >
> >> > https://github.com/torvalds/linux/blob/v6.11-rc3/drivers/ata/pata_macio.c#L544
> >> >
> >> > It seems that the
> >> > while (sg_len) loop does not play nice with the new .max_segment_size.
> >> 
> >> Right, but only for 4KB kernels for some reason. Is there some limit
> >> elsewhere that prevents the bug tripping on 64KB kernels, or is it just
> >> luck that no one has hit it?
> >
> > Have your tried running fio (flexible I/O tester), with reads with a very
> > large block sizes?
> >
> > I would be surprised if it isn't possible to trigger the same bug with
> > 64K page size.
> >
> > max segment size = 64K
> > MAX_DCMDS = 256
> > 256 * 64K = 16 MiB
> > What happens if you run fio with a 16 MiB blocksize?
> >
> > Something like:
> > $ sudo fio --name=test --filename=/dev/sdX --direct=1 --runtime=60 --ioengine=io_uring --rw=read --iodepth=4 --bs=16M
> 
> Nothing interesting happens, fio succeeds.
> 
> The largest request that comes into pata_macio_qc_prep() is 1280KB,
> which results in 40 DMA list entries.
> 
> I tried with a larger block size but it doesn't change anything. I guess
> there's some limit somewhere else in the stack?
> 
> That was testing on qemu, but I don't think it should matter?
> 
> I guess there's no way to run the fio test against a file, ie. without a
> raw partition? My real G5 doesn't have any spare disks/partitions in it.


You can definitely run fio against a file.

e.g.
$ dd if=/dev/random of=/tmp/my_file bs=1M count=1024

$ sudo fio --name=test --filename=/tmp/my_file --direct=1 --runtime=60 --ioengine=io_uring --rw=read --iodepth=4 --bs=16M


Perhaps try with 32M block size, so that it is larger than
max segment size = 64K
MAX_DCMDS = 256
256 * 64K = 16 MiB

Perhaps also try with and without --direct.
It could be interesting to use the page cache if you do --rw=readwrite
that might possibly result in larger bios.


Kind regards,
Niklas