Re: [PATCH 0/6] mempolicy memory corruption fixlet

[KOSAKI Motohiro]

(6/5/12 3:17 PM), Andrew Morton wrote:
> On Tue, 5 Jun 2012 12:02:25 -0700
> Linus Torvalds<torvalds@xxxxxxxxxxxxxxxxxxxx>  wrote:
>
> > I'm coming back to this email thread, because I didn't apply the
> > series due to all the ongoing discussion and hoping that somebody
> > would put changelog fixes and ack notices etc together.
> >
> > I'd also really like to know that the people who saw the problem that
> > caused the current single patch (that this series reverts) would test
> > the whole series. Maybe that happened and I didn't notice it in the
> > threads, but I don't think so.

I'm not surprised this. If many people are interesting to review this area,
mempolicy wouldn't have break so a lot.


> > In fact, right now I'm assuming that the series will eventually come
> > to me through Andrew. Andrew, correct?
>
> yup.
>
> I expect there will be a v2 series (at least).  It's unclear what
> we'll be doing with [2/6]: whether the patch will be reworked, or
> whether Andi misunderstood its effects?

Maybe because Andi didn't join bug fix works in this area for several years?


Currently, mbind(2) is completely broken. A primary role of mbind(2) is to
update memory policy of some vmas and Mel's fix remvoed it. Then, mbind is
almostly no-op. it's a regression.

I'm not clear which point you seems unclear. So, let's repeat a description of
[2/6].

There are two problem now, alloc_pages_vma() has strong and wrong assumption.
vma->policy never have MPOL_F_SHARED and shared_policy->policy must have it.
And, cpusets rebinding ignore mpol->refcnt and updates it forcibly.

The final point is to implement cow. But for it, we need rewrite mpol->rebind
family completely. It doesn't fit for 3.5 timeframe.


The downside of patch [2/6] is very small. because,

A memplicy is only shared three cases, 1) mbind() updates multiple
vmas 2) mbind() updates shmem vma 3) A shared policy splits into two regions
by a part region update.

All of them are rare. Because nobody hit kernel panic until now. Then I don't
think my patch increase memory footprint.


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