linux-kernel - Re: [PATCH 10/16] mm/hugetlb: update hugetlbfs to use mmap_prepare, mmap

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Message-ID: <f81fe6d4-43d2-461d-81b9-032a590f5b22@lucifer.local>
Date: Mon, 8 Sep 2025 14:37:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
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Subject: Re: [PATCH 10/16] mm/hugetlb: update hugetlbfs to use mmap_prepare,
 mmap_complete

On Mon, Sep 08, 2025 at 10:11:21AM -0300, Jason Gunthorpe wrote:
> On Mon, Sep 08, 2025 at 12:10:41PM +0100, Lorenzo Stoakes wrote:
> > @@ -151,20 +123,55 @@ static int hugetlbfs_file_mmap(struct file *file, struct vm_area_struct *vma)
> >  		vm_flags |= VM_NORESERVE;
> >
> >  	if (hugetlb_reserve_pages(inode,
> > -				vma->vm_pgoff >> huge_page_order(h),
> > -				len >> huge_page_shift(h), vma,
> > -				vm_flags) < 0)
> > +			vma->vm_pgoff >> huge_page_order(h),
> > +			len >> huge_page_shift(h), vma,
> > +			vm_flags) < 0) {
>
> It was split like this because vma is passed here right?
>
> But hugetlb_reserve_pages() doesn't do much with the vma:
>
> 	hugetlb_vma_lock_alloc(vma);
> [..]
> 	vma->vm_private_data = vma_lock;
>
> Manipulates the private which should already exist in prepare:
>
> Check non-share a few times:
>
> 	if (!vma || vma->vm_flags & VM_MAYSHARE) {
> 	if (vma && !(vma->vm_flags & VM_MAYSHARE) && h_cg) {
> 	if (!vma || vma->vm_flags & VM_MAYSHARE) {
>
> And does this resv_map stuff:
>
> 		set_vma_resv_map(vma, resv_map);
> 		set_vma_resv_flags(vma, HPAGE_RESV_OWNER);
> [..]
> 	set_vma_private_data(vma, (unsigned long)map);
>
> Which is also just manipulating the private data.
>
> So it looks to me like it should be refactored so that
> hugetlb_reserve_pages() returns the priv pointer to set in the VMA
> instead of accepting vma as an argument. Maybe just pass in the desc
> instead?

Well hugetlb_vma_lock_alloc() does:

	vma_lock->vma = vma;

Which we cannot do in prepare.

This is checked in hugetlb_dup_vma_private(), and obviously desc is not a stable
pointer to be used for comparing anything.

I'm also trying to do the minimal changes I can here, I'd rather not majorly
refactor things to suit this change if possible.

>
> Then no need to introduce complete. I think it is probably better to
> try to avoid using complete except for filling PTEs..

I'd rather do that yes. hugetlbfs is the exception to many rules, unfortunately.

>
> Jason

Cheers, Lorenzo