On Wed, Aug 21, 2024 at 9:24 AM Pedro Falcato <pedro.falcato@xxxxxxxxx> wrote:
>
> On Wed, Aug 21, 2024 at 5:16 PM Jeff Xu <jeffxu@xxxxxxxxxxxx> wrote:
> >
> > On Fri, Aug 16, 2024 at 5:18 PM Pedro Falcato <pedro.falcato@xxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > > 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
