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?
