Re: [PATCH] ublk: fix maple tree lockdep warning and unpin under spinlock

[Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>]

On Apr 22, 2026 / 08:08, Ming Lei wrote:
> From: Ming Lei <ming.lei@redhat.com>
> 
> Fix two issues in the shmem buffer maple tree usage:
> 
> 1) ublk_buf_cleanup() iterates the tree with mas_for_each() without
>    holding rcu_read_lock or mas_lock, triggering a lockdep splat on
>    CONFIG_PROVE_RCU kernels. Add mas_lock/unlock around the iteration.
> 
> 2) __ublk_ctrl_unreg_buf() calls unpin_user_pages() under mas_lock
>    (a spinlock). unpin_user_pages can be expensive for large buffers
>    and may take additional locks if folio refcount drops to zero.
>    Restructure to drop mas_lock before unpinning, re-acquiring it
>    to continue iteration.
> 
> Both functions now use the same pattern: erase under lock, drop lock,
> unpin and free, re-lock to continue. Extract ublk_unpin_range_pages()
> helper to share the page unpinning loop.
> 
> Reported-by: Jens Axboe <axboe@kernel.dk>
> Closes: https://lore.kernel.org/linux-block/0349d72d-dff8-4f9f-b448-919fa5ae96da@kernel.dk/
> Cc: Liam R. Howlett <liam.howlett@oracle.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>

I observed the same WARN at blktests test cases block/033 and ublk/*. I
confirmed this patch avoids the WARN and makes the test cases pass. Thanks!

Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>

I also bisected and confirmed that the commit below is the failure trigger.
Adding a Fixes tag might add a bit of value.

Fixes: 5e864438e285 ("ublk: replace xarray with IDA for shmem buffer index allocation")