Re: [PATCH v10 0/5] Introduce mseal

Jeff Xu
2024-04-20T01:23:18Z

On Fri, Apr 19, 2024 at 10:59 AM Pedro Falcato <pedro.falcato@gmail.com> wrote:
>
> On Fri, Apr 19, 2024 at 2:22 AM Jeff Xu <jeffxu@chromium.org> wrote:
> > The overhead is likely to grow linearly with the number of VMA, since
> > it takes time to retrieve VMA's metadata.
> >
> > Let's use one data sample to look at impact:
> >
> > Test: munmap 1000 memory range, each memory range has 1 VMA
> >
> > syscall__ vmas t t_mseal delta_ns per_vma %
> > munmap__ 1 909 944 35 35 104%
> >
> > For those 1000 munmap calls, sealing adds 35000 ns in total, or 35 ns per call.
>
> Have you tried to spray around some likely() and unlikely()s? Does
> that make a difference? I'm thinking that sealing VMAs will be very
> rare, and mprotect/munmapping them is probably a programming error
> anyway, so the extra branches in the mprotect/munmap/madvice (etc)
> should be a nice target for some branch annotation.
>
Most cost will be in locating the node in the maple tree that stores
the VMAs, branch annotation is not possible there.

We could put unlikely() in the can_modify_mm check, I suspect it
won't make a measurable difference in the real-world. On the other
hand, this also causes no harm, and existing mm code uses
unlikely/likely in a lot of places, so why not.

I will send a follow-up patch in the next few days.

Thanks
-Jeff
> --
> Pedro