linux-kernel - Re: [PATCH drm-next 1/2] vmalloc: Add atomic

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Message-ID: <Z8nEqDQhjU-Ly8Js@phenom.ffwll.local>
Date: Thu, 6 Mar 2025 16:52:08 +0100
From: Simona Vetter <simona.vetter@...ll.ch>
To: Jocelyn Falempe <jfalempe@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>,
	Ryosuke Yasuoka <ryasuoka@...hat.com>,
	maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
	tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch,
	kraxel@...hat.com, gurchetansingh@...omium.org, olvaffe@...il.com,
	akpm@...ux-foundation.org, urezki@...il.com, hch@...radead.org,
	dmitry.osipenko@...labora.com, dri-devel@...ts.freedesktop.org,
	linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
	linux-mm@...ck.org
Subject: Re: [PATCH drm-next 1/2] vmalloc: Add atomic_vmap

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 ...

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
-- 
Simona Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch