linux-kernel - Re: [PATCH v3 2/7] mm/munmap: Replace can_modify_mm with can_modify

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Message-ID: <CABi2SkUZ-OQFCNZcc0OMN_4YV8NoqrRu2HGEeFms+Ty6Bnofdg@mail.gmail.com>
Date: Wed, 21 Aug 2024 09:33:06 -0700
From: Jeff Xu <jeffxu@...omium.org>
To: Pedro Falcato <pedro.falcato@...il.com>, rientjes@...gle.com
Cc: Andrew Morton <akpm@...ux-foundation.org>, "Liam R. Howlett" <Liam.Howlett@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Shuah Khan <shuah@...nel.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, oliver.sang@...el.com, 
	torvalds@...ux-foundation.org, Michael Ellerman <mpe@...erman.id.au>, 
	Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 2/7] mm/munmap: Replace can_modify_mm with can_modify_vma

On Wed, Aug 21, 2024 at 9:24 AM Pedro Falcato <pedro.falcato@...il.com> wrote:
>
> On Wed, Aug 21, 2024 at 5:16 PM Jeff Xu <jeffxu@...omium.org> wrote:
> >
> > On Fri, Aug 16, 2024 at 5:18 PM Pedro Falcato <pedro.falcato@...il.com> wrote:
> > >
> > > We were doing an extra mmap tree traversal just to check if the entire
> > > range is modifiable. This can be done when we iterate through the VMAs
> > > instead.
> > >
> > > Signed-off-by: Pedro Falcato <pedro.falcato@...il.com>
> > > ---
> > >  mm/mmap.c | 11 +----------
> > >  mm/vma.c  | 19 ++++++++++++-------
> > >  2 files changed, 13 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index 3af256bacef3..30ae4cb5cec9 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -1740,16 +1740,7 @@ int do_vma_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> > >                 unsigned long start, unsigned long end, struct list_head *uf,
> > >                 bool unlock)
> > >  {
> > > -       struct mm_struct *mm = vma->vm_mm;
> > > -
> > > -       /*
> > > -        * Check if memory is sealed, prevent unmapping a sealed VMA.
> > > -        * can_modify_mm assumes we have acquired the lock on MM.
> > > -        */
> > > -       if (unlikely(!can_modify_mm(mm, start, end)))
> > > -               return -EPERM;
> > Another approach to improve perf  is to clone the vmi (since it
> > already point to the first vma), and pass the cloned vmi/vma into
> > can_modify_mm check, that will remove the cost of re-finding the first
> > VMA.
> >
> > The can_modify_mm then continues from cloned VMI/vma till the end of
> > address range, there will be some perf cost there.  However,  most
> > address ranges in the real world are within a single VMA,  in
> > practice, the perf cost is the same as checking the single VMA, 99.9%
> > case.
> >
> > This will help preserve the nice sealing feature (if one of the vma is
> > sealed, the entire address range is not modified)
>
> Please drop it. No one wants to preserve this. Everyone is in sync
> when it comes to the solution except you.

Still, this is another option that will very likely address the perf issue.

-Jeff

>
> --
> Pedro