Re: [syzbot] [iomap?] kernel BUG in folio_end_read (2)

[John Ogness]

On 2025-11-04, Petr Mladek <pmladek@xxxxxxxx> wrote:
> Adding John into Cc.

Thanks.

> It rather looks like an internal bug in the printk_ringbuffer code.
> And there is only one recent patch:
>
>    https://patch.msgid.link/20250905144152.9137-2-d-tatianin@xxxxxxxxxxxxxx
>
> The scenario leading to the WARN() is not obvious to me. But the patch
> touched this code path. So it is a likely culprit. I have to think
> more about it.

I have been digging into this all day and I can find no explanation.

The patch you refer to brings a minor semantic change: is_blk_wrapped()
returns false if begin_lpos and next_lpos are the same, whereas before
we would have true. However, these values are not allowed to be the same
(except for the data-less special case values).

> Anyway, I wonder if the WARNING is reproducible and if it happens even after
> reverting the commit 67e1b0052f6bb82be84e3 ("printk_ringbuffer: don't
> needlessly wrap data blocks around")

Note that a quick search on lore shows another similar report:

https://lore.kernel.org/all/69078fb6.050a0220.29fc44.0029.GAE@xxxxxxxxxx/

We may want to revert the commit until we can take a closer look at
this.

I will divert my energies from code-reading to trying to reproduce this.

John