Re: [PATCH drm-next 1/2] vmalloc: Add atomic_vmap

[Matthew Wilcox <willy@infradead.org>]

On Thu, Mar 06, 2025 at 12:25:53AM +0900, Ryosuke Yasuoka wrote:
> Some drivers can use vmap in drm_panic, however, vmap is sleepable and
> takes locks. Since drm_panic will vmap in panic handler, atomic_vmap
> requests pages with GFP_ATOMIC and maps KVA without locks and sleep.

In addition to the implicit GFP_KERNEL allocations Vlad mentioned, how
is this supposed to work?

> +	vn = addr_to_node(va->va_start);
> +
> +	insert_vmap_area(va, &vn->busy.root, &vn->busy.head);

If someone else is holding the vn->busy.lock because they're modifying the
busy tree, you'll corrupt the tree.  You can't just say "I can't take a
lock here, so I won't bother".  You need to figure out how to do something
safe without taking the lock.  For example, you could preallocate the
page tables and reserve a vmap area when the driver loads that would
then be usable for the panic situation.  I don't know that we have APIs
to let you do that today, but it's something that could be added.