Re: [PATCH 2/5] drm/nouveau/uvmm: Allow larger pages

[Lyude Paul <lyude@redhat.com>]

On Thu, 2025-10-23 at 13:14 +0300, Mohamed Ahmed wrote:
> The other thing making me hesitant of depending on
> nouveau_bo_fixup_align() is that VM_BIND is entirely client controlled
> and there isn't really (at least as far as I understand) way for the
> bo_fixup_align() path to have enough info to e.g. work around the
> "client allocates size and binds to address not aligned to that size"
> issue (likely the reason for hitting the mismatch case. this didn't
> show in the older kernel versions because everything was forced to 4K
> anyways).

Gotcha, yeah - Mary's explanation convinced me. I think then as long as we
convert to using the math64 functions, drop the WARN_ON and document it like
Mary suggested then this is probably fine and we can leave
nouveau_bo_fixup_align() the way it is.

One other change we should consider making though: can we make page_shift 32
bit? A page shift of 32 would give us 2GB pages and I think that sounds way
larger then anything we'd expect to encounter. Plus, we could just warn if we
get a page shift larger then 32 bit and fail the ioctl. 64bit % 32bit should
be faster and at least alleviate some of the overhead from the math here.

> 
> On Thu, Oct 23, 2025 at 12:39 AM Mary Guillemard <mary@mary.zone> wrote:
> > 
> > On Wed, Oct 22, 2025 at 10:56 PM Danilo Krummrich <dakr@kernel.org> wrote:
> > > 
> > > On 10/22/25 12:16 PM, Mohamed Ahmed wrote:
> > > > Pinging again re: review and also was asking if we can revert the
> > > > select_page_shift() handling back to v1 behavior with a fall-back
> > > > path, as it looks like there are some cases where
> > > > nouveau_bo_fixup_align() isn't enough;
> > > > https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36450#note_3159199.
> > > 
> > > I don't think we should add a fallback for something that is expected to be
> > > sufficient.
> > > 
> > > Instead we should figure out in which exact case the WARN_ON() was hit and why.
> > 
> > The reason I wrote this code initially was to handle addresses
> > provided by userspace that aren't aligned to the page size selected
> > during BO creation.
> > This is something I did trigger when typing this patch initially with
> > my distro provided version of mesa (likely 25.0.x but it has been a
> > while)
> > Thomas Andersen also confirmed on nouveau irc channel that he did hit
> > this case with an old version of NVK and this patchset.
> > 
> > I think we could just remove the WARN_ON and properly document that
> > this was previously allowed and is there for backward compatibility.
> > 
> > Regards,
> > Mary Guillemard

-- 
Cheers,
 Lyude Paul (she/her)
 Senior Software Engineer at Red Hat

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