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

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Message-ID: <20250908135240.GI616306@nvidia.com>
Date: Mon, 8 Sep 2025 10:52:40 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 02:37:44PM +0100, Lorenzo Stoakes wrote:
> 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.

Okay, just doing that in commit would be appropriate then
 
> 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.

It doesn't look like a bit refactor, pass vma desc into
hugetlb_reserve_pages(), lift the vma_lock set out

Jason