Re: [PATCH v3 15/16] mm/mmap: Use vms accounted pages in mmap_region()

[Suren Baghdasaryan]

On Mon, Jul 8, 2024 at 5:43 AM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> On Thu, Jul 04, 2024 at 02:27:17PM GMT, Liam R. Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> >
> > Change from nr_pages variable to vms.nr_accounted for the charged pages
> > calculation.  This is necessary for a future patch.
> >
> > This also avoids checking security_vm_enough_memory_mm() if the amount
> > of memory won't change.
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> > Cc: Kees Cook <kees@xxxxxxxxxx>
> > ---
> >  mm/mmap.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index b14da6bd257f..b2de26683903 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -2980,6 +2980,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >       } else {
> >               /* Minimal setup of vms */
> >               vms.nr_pages = 0;
> > +             vms.nr_accounted = 0;
>
> This kind of highlights my concern about only setting some vms fields, now we
> have to remember to change this in the right place or happen to know that
> init_vma_munmap() will be otherwise invoked.
>
> >               next = vma_next(&vmi);
> >               prev = vma_prev(&vmi);
> >               if (prev)
> > @@ -2991,9 +2992,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >        */
> >       if (accountable_mapping(file, vm_flags)) {
> >               charged = pglen;
> > -             charged -= nr_accounted;
> > -             if (security_vm_enough_memory_mm(mm, charged))
> > +             charged -= vms.nr_accounted;
> > +             if (charged && security_vm_enough_memory_mm(mm, charged))
> >                       goto abort_munmap;
> > +
> >               vms.nr_accounted = 0;
>
> Is setting this to zero really needed here? We may be done with this, but if the
> vms value represents the 'unmap state' of this range, surely the number of
> accountable pages remains the same?
>
> >               vm_flags |= VM_ACCOUNT;
> >       }
> > --
> > 2.43.0
> >
>
> At this point nr_accounted is no longer used, but I'm guessing a follow up patch
> will remove this? :)

IMHO this and the next patch can be combined to remove this confusion.
They are both rather small, so would not be a big deal.


>
> I was wondering why you used that given the gather function also separately
> calculates it, but I guess this answers that!
>
> Generally this looks good to me, so:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>