linux-kernel - Re: [RFC PATCH 2/6] mm/mmap: Abstract vma clean up from exit

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Message-ID: <ft4brzgr6jitmyfs4jplbokpqoz2z2ejsudfrezuke2oqygbs5@b5i3zaxgxrta>
Date: Wed, 3 Sep 2025 15:56:03 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: David Hildenbrand <david@...hat.com>, maple-tree@...ts.infradead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Charan Teja Kalla <quic_charante@...cinc.com>,
        shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com,
        bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [RFC PATCH 2/6] mm/mmap: Abstract vma clean up from exit_mmap()

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250819 14:38]:
> On Fri, Aug 15, 2025 at 03:10:27PM -0400, Liam R. Howlett wrote:
> > Create the new function tear_down_vmas() to remove a range of vmas.
> > exit_mmap() will be removing all the vmas.
> >
> > This is necessary for future patches.
> >
> > No functional changes intended.
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> 
> This function is pure and complete insanity, but this change looks
> good. Couple nits below.
> 
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> 
> > ---
> >  mm/mmap.c | 37 ++++++++++++++++++++++++-------------
> >  1 file changed, 24 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index c4c315b480af7..0995a48b46d59 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1250,6 +1250,29 @@ int vm_brk_flags(unsigned long addr, unsigned long request, vm_flags_t vm_flags)
> >  }
> >  EXPORT_SYMBOL(vm_brk_flags);
> >
> > +static inline
> > +unsigned long tear_down_vmas(struct mm_struct *mm, struct vma_iterator *vmi,
> > +		struct vm_area_struct *vma, unsigned long max)
> > +{
> > +	unsigned long nr_accounted = 0;
> > +	int count = 0;
> > +
> > +	mmap_assert_write_locked(mm);
> > +	vma_iter_set(vmi, vma->vm_end);
> > +	do {
> > +		if (vma->vm_flags & VM_ACCOUNT)
> > +			nr_accounted += vma_pages(vma);
> > +		vma_mark_detached(vma);
> > +		remove_vma(vma);
> > +		count++;
> > +		cond_resched();
> > +		vma = vma_next(vmi);
> > +	} while (vma && vma->vm_end <= max);
> > +
> > +	BUG_ON(count != mm->map_count);
> 
> Can we make this a WARN_ON() or WARN_ON_ONCE() while we're at it?

Sure!

> 
> > +	return nr_accounted;
> > +}
> > +
> >  /* Release all mmaps. */
> >  void exit_mmap(struct mm_struct *mm)
> >  {
> > @@ -1257,7 +1280,6 @@ void exit_mmap(struct mm_struct *mm)
> >  	struct vm_area_struct *vma;
> >  	unsigned long nr_accounted = 0;
> 
> No need to initialise this to 0 any more.

There is a goto label below that skips calling the tear down, so this is
still needed.

Thanks,
Liam