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

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Message-ID: <edd2b2d1-fdd7-4e11-9024-03098da78dd1@lucifer.local>
Date: Fri, 23 May 2025 11:04:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Ricardo Cañuelo Navarro <rcn@...lia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Pedro Falcato <pfalcato@...e.de>, revest@...gle.com,
        kernel-dev@...lia.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH] mm: fix copy_vma() error handling for hugetlb mappings

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!