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

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Mar 06, 2025 at 02:24:51PM +0100, Jocelyn Falempe wrote:
> On 06/03/2025 05:52, Matthew Wilcox wrote:
> > 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.
> > 
> Regarding the lock, it should be possible to use the trylock() variant, and
> fail if the lock is already taken. (In the panic handler, only 1 CPU remain
> active, so it's unlikely the lock would be released anyway).
> 
> If we need to pre-allocate the page table and reserve the vmap area, maybe
> it would be easier to just always vmap() the primary framebuffer, so it can
> be used in the panic handler?
> 
We can reserve a vmap space for ATOMIC or NOWAIT allocations. As for PTE
part, we can also populate reserved space, because after that operation
those are never get released.

The question is how many users need this. As for this particular case i
am in line with Jocelyn Falempe. Allocate for DRM and write on panic.

--
Uladzislau Rezki