Re: [REGRESSION] x86/hugetlb: AMD F15h VA alignment offset breaks MAP_HUGETLB alignment

[Oscar Salvador (SUSE)]

On Wed, May 27, 2026 at 02:04:10PM -0700, Dave Hansen wrote:
> On 5/27/26 11:28, Oscar Salvador (SUSE) wrote:
> >   	if (filp) {
> >   		info.align_mask = get_align_mask(filp);
> >  -		info.align_offset += get_align_bits();
> >  +		/*
> >  +		 * Hugepages must remain hugepage-aligned, so skip adding an offset
> >  +		 * in case we enabled 'align_va_addr'.
> >  +		 */
> >  +		if (!is_file_hugepages(filp))
> >  +			info.align_offset += get_align_bits();
> >   	}
> 
> That's a good hack to show the scope of the problem.

Haha, do not worry, I myself have 0 interestin spreading hugetlb-specific
code around (on the contrary), but I wanted to proof the point.

> 
> But I'd really rather this be dealt with in the arch-independent code,
> not by adding hugetlb hacks to arch code. It isn't even clear to me what
> exactly goes wrong when you set a tiny ->align_offset and have a larger
> ->align_mask. Shouldn't the tiny offset just get masked off?
> 
>         gap += (info->align_offset - gap) & info->align_mask;

I would assume so, but something is definitely going in with the calculation,
as I could reproduce this if I artificially set align_offset to 0x1000 for
hugetlb mappings.

I will find some time today to have a deep look.


-- 
Oscar Salvador
SUSE Labs