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.
--
Pedro
