On Wed, Nov 26, 2025 at 09:31:46AM +0900, Alexandre Courbot wrote: > On Tue Nov 25, 2025 at 11:59 PM JST, Alice Ryhl wrote: > > On Tue, Nov 25, 2025 at 3:55 PM Alexandre Courbot <[email protected]> > > wrote: > >> > >> On Tue Nov 25, 2025 at 11:41 PM JST, Alice Ryhl wrote: > >> > On Tue, Nov 25, 2025 at 3:29 PM Alexandre Courbot <[email protected]> > >> > wrote: > >> >> > >> >> On Fri Nov 21, 2025 at 1:04 PM JST, bshephar wrote: > >> >> > Use page::page_align for GEM object memory allocation to ensure the > >> >> > allocation is page aligned. This ensures that the allocation is page > >> >> > aligned with the system in cases where 4096 is not the default. > >> >> > For example on 16k or 64k aarch64 systems this allocation should be > >> >> > aligned accordingly. > >> >> > > >> >> > Signed-off-by: Brendan Shephard <[email protected]> > >> >> > --- > >> >> > drivers/gpu/drm/nova/gem.rs | 11 ++++++++--- > >> >> > 1 file changed, 8 insertions(+), 3 deletions(-) > >> >> > > >> >> > diff --git a/drivers/gpu/drm/nova/gem.rs b/drivers/gpu/drm/nova/gem.rs > >> >> > index 2760ba4f3450..a07e922e25ef 100644 > >> >> > --- a/drivers/gpu/drm/nova/gem.rs > >> >> > +++ b/drivers/gpu/drm/nova/gem.rs > >> >> > @@ -3,6 +3,10 @@ > >> >> > use kernel::{ > >> >> > drm, > >> >> > drm::{gem, gem::BaseObject}, > >> >> > + page::{ > >> >> > + page_align, > >> >> > + PAGE_SIZE, // > >> >> > + }, > >> >> > prelude::*, > >> >> > sync::aref::ARef, > >> >> > }; > >> >> > @@ -27,12 +31,13 @@ fn new(_dev: &NovaDevice, _size: usize) -> impl > >> >> > PinInit<Self, Error> { > >> >> > impl NovaObject { > >> >> > /// Create a new DRM GEM object. > >> >> > pub(crate) fn new(dev: &NovaDevice, size: usize) -> > >> >> > Result<ARef<gem::Object<Self>>> { > >> >> > - let aligned_size = size.next_multiple_of(1 << 12); > >> >> > - > >> >> > - if size == 0 || size > aligned_size { > >> >> > + // Check for 0 size or potential usize overflow before > >> >> > calling page_align > >> >> > + if size == 0 || size > usize::MAX - PAGE_SIZE + 1 { > >> >> > >> >> `PAGE_SIZE` here is no more correct than the hardcoded `1 << 12` - well, > >> >> I'll admit it looks better as a placeholder. :) But the actual alignment > >> >> will eventually be provided elsewhere. > >> > > >> > What about kernels with 16k pages? > >> > >> The actual alignment should IIUC be a mix of the GPU and kernel's > >> requirements (GPU can also use a different page size). So no matter what > >> we pick right now, it won't be great but you are right that PAGE_SIZE > >> will at least accomodate the kernel. > > > > In that case, is PAGE_SIZE not the wrong constant? What's the actually > > correct constant here? > > > >> >> > return Err(EINVAL); > >> >> > } > >> >> > > >> >> > + let aligned_size = page_align(size); > >> >> > >> >> `page_align` won't panic on overflow, but it will still return an > >> >> invalid size. This is a job for `kernel::ptr::Alignment`, which let's > >> >> you return an error when an overflow occurs. > >> > > >> > The Rust implementation of page_align() is implemented as (addr + > >> > (PAGE_SIZE - 1)) & PAGE_MASK, which definitely will panic on overflow > >> > if the appropriate config options are enabled. > >> > >> That's right, I skimmed its code too fast. ^_^; All the more reason to > >> use `Alignment`. > > > > Alignment stores values that are powers of two, not multiples of PAGE_SIZE. > > Isn't PAGE_SIZE always a power of two though?
Yes it is. Maybe you can elaborate on how you wanted to use Alignment? It sounds like you have something different in mind than what I thought. Alice
