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

Peter Xu
2025-06-13T18:31:28Z

On Fri, Jun 13, 2025 at 01:00:20PM -0300, Jason Gunthorpe wrote:
> 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

Sure thing.

>
> > > 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.

Good to learn the traps, and I also wasn't aware of the helpers. I'll
switch to that, thanks!

>
> > +/**
> > + * 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"

Sure.

>
> > + * @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"

This is not true though when pgoff isn't aligned..

For example, an allocation with (len=32M, size=2M, pgoff=1M) will return an
address that is N*2M+1M, so that starting from pgoff=2M it'll be completely
aligned. In this case the returned mmap() address must not be aligned to
make it happen, and the range within pgoff=1M-2M will be mapped with 4K.

--
Peter Xu