Re: [PATCH 3/5] mm: Rename __thp_get_unmapped_area to mm_get_unmapped_area_aligned

Jason Gunthorpe
2025-06-13T16:00:31Z

On Fri, Jun 13, 2025 at 11:13:58AM -0400, Peter Xu wrote:
> > I didn't intuitively guess how it works or why there are two
> > length/size arguments. It seems to have an exciting return code as
> > well.
> >
> > I suppose size is the alignment target? Maybe rename the parameter too?
>
> Yes, when the kdoc is there it'll be more obvious. So far "size" is ok to
> me, but if you have better suggestion please shoot - whatever I came up
> with so far seems to be too long, and maybe not necessary when kdoc will be
> available too.

I would call it align not size

> > For the purposes of VFIO do we need to be careful about math overflow here:
> >
> > loff_t off_end = off + len;
> > loff_t off_align = round_up(off, size);
> >
> > ?
>
> IIUC the 1st one was covered by the latter check here:
>
> (off + len_pad) < off
>
> Indeed I didn't see what makes sure the 2nd won't overflow.

I'm not sure the < tests are safe in this modern world. I would use
the overflow helpers directly and remove the < overflow checks.

> +/**
> + * mm_get_unmapped_area_aligned - Allocate an aligned virtual address
> + * @filp: file target of the mmap() request
> + * @addr: hint address from mmap() request
> + * @len: len of the mmap() request
> + * @off: file offset of the mmap() request
> + * @flags: flags of the mmap() request
> + * @size: the size of alignment the caller requests

Just "the alignment the caller requests"

> + * @vm_flags: the vm_flags passed from get_unmapped_area() caller
> + *
> + * This function should normally be used by a driver's specific
> + * get_unmapped_area() handler to provide a properly aligned virtual
> + * address for a specific mmap() request. The caller should pass in most
> + * of the parameters from the get_unmapped_area() request, but properly
> + * specify @size as the alignment needed.

.. "The function willl try to return a VMA starting address such that
ret % size == 0"

Jason