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

[Ryosuke Yasuoka <ryasuoka@redhat.com>]

On Fri, Mar 7, 2025 at 4:55 PM Jocelyn Falempe <jfalempe@redhat.com> wrote:
>
> On 06/03/2025 16:52, Simona Vetter wrote:
> > 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?
> >
> > Yeah I really don't like the idea of creating some really brittle one-off
> > core mm code just so we don't have to vmap a buffer unconditionally. I
> > think even better would be if drm_panic can cope with non-linear buffers,
> > it's entirely fine if the drawing function absolutely crawls and sets each
> > individual byte ...
>
> It already supports some non-linear buffer, like Nvidia block-linear:
> https://elixir.bootlin.com/linux/v6.13.5/source/drivers/gpu/drm/nouveau/dispnv50/wndw.c#L606
>
> And I've also sent some patches to support Intel's 4-tile and Y-tile format:
> https://patchwork.freedesktop.org/patch/637200/?series=141936&rev=5
> https://patchwork.freedesktop.org/patch/637202/?series=141936&rev=5
>
> Hopefully Color Compression can be disabled on intel's GPU, otherwise
> that would be a bit harder to implement than tiling.
>
> >
> > The only thing you're allowed to do in panic is try_lock on a raw spinlock
> > (plus some really scare lockless tricks), imposing that on core mm sounds
> > like a non-starter to me.
> >
> > Cheers, Sima
>

Thank you all for your comments.
I understand adding atomic_vmap is not possible as vmalloc is not compatible
with GFP_ATOMIC. I'll re-implement this by pre-allocating the page table and
reserve the vmap area while the kernel is alive. It'll might be
allocated in driver
codes so maybe I don't need to add any features in core mm code.

Best regards,
Ryosuke