On Wed, Dec 8, 2021 at 4:05 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Wed, Dec 08, 2021 at 08:35:34PM +0000, Pasha Tatashin wrote:
> > It is hard to root cause _refcount problems, because they usually
> > manifest after the damage has occurred. Yet, they can lead to
> > catastrophic failures such memory corruptions. There were a number
> > of refcount related issues discovered recently [1], [2], [3].
> >
> > Improve debugability by adding more checks that ensure that
> > page->_refcount never turns negative (i.e. double free does not
> > happen, or free after freeze etc).
> >
> > - Check for overflow and underflow right from the functions that
> > modify _refcount
> > - Remove set_page_count(), so we do not unconditionally overwrite
> > _refcount with an unrestrained value
> > - Trace return values in all functions that modify _refcount
>
Hi Matthew,
Thank you for looking at this series.
> You're doing a lot more atomic instructions with these patches.
This is not exactly so. There are no *more* atomic instructions. There
are, however, different atomic instructions:
For example: atomic_add() becomes atomic_fetch_add()
On x86 it is:
atomic_add:
lock add %eax,(%rsi)
atomic_fetch_add:
lock xadd %eax,(%rsi)
On ARM64, I believe the same CAS instruction is used for both.
Have you
> done any performance measurements with these patches applied and debug
> disabled?
Yes, I have done some performance tests exactly as you described with
CONFIG_DEBUG_VM disabled and these patches applied.
I tried: hackbench, unixbench, and a few more benchmarks; I did not
see any performance difference.
> I'm really not convinced it's worth closing
> one-instruction-wide races of this kind when they are "shouldn't ever
> happen" situations. If the debugging will catch the problem in 99.99%
> of cases and miss 0.01% without using atomic instructions, that seems
> like a better set of tradeoffs than catching 100% of problems by using
> the atomic instructions.
I think we should relax the precise catching of bugs only if there is
indeed a measurable performance impact. The problem is that if there
is a __refcount bug, the security consequences are dire as it may lead
to leaking memory from one process to another.
Thanks,
Pasha
