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

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Message-ID: <CAKbZUD1d5hXWUPVpUzPo=M33_eo9_sdMq6nVg-jd=-O-CpFjsg@mail.gmail.com>
Date: Wed, 21 Aug 2024 17:23:59 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Jeff Xu <jeffxu@...omium.org>
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 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.

-- 
Pedro