Re: [PATCH] mm: fix copy_vma() error handling for hugetlb mappings

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Fri, May 23, 2025 at 11:00:40AM +0100, Lorenzo Stoakes wrote:
[snip]
> > ---
> >  mm/vma.c | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 839d12f02c885d3338d8d233583eb302d82bb80b..9d9f699ace977c9c869e5da5f88f12be183adcfb 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -1834,6 +1834,8 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
> >  	return new_vma;
> >
> >  out_vma_link:
> > +	if (is_vm_hugetlb_page(new_vma))
> > +		clear_vma_resv_huge_pages(new_vma);
>
> So,
>
> Could you implement this slightly differently please? We're duplicating
> this code now, so I think this should be in its own function with a copious
> comment.
>
> Something like:
>
> static void fixup_hugetlb_reservations(struct vm_area_struct *vma)
> {
> 	if (is_vm_hugetlb_page(new_vma))
> 		clear_vma_resv_huge_pages(new_vma);
> }
>
> And call this from here and also in copy_vma_and_data().

Sorry, I wasn't clear, Can you please ensure that you put a chonking big comment
on this function too please? Something that explains the situation, e.g.:

/*
 * For hugetlb, mremap() is an odd edge case - while the VMA copying is
 * performed, we permit both the old and new VMAs to reference the same
 * reservation.
 *
 * We fix this up after the operation succeeds, or if a newly allocated VMA
 * is closed as a result of a failure to allocate memory.
 */

[snip]

Thanks!