Re: [PATCH 0/3] ksm: write protect pages from inside ksm

[Izik Eidus]

Izik Eidus wrote:
> Izik Eidus wrote:
> > Hugh Dickins wrote:
> > > On Sat, 13 Jun 2009, Izik Eidus wrote:
> > >  
> > > > Hugh, so untill here we are sync,
> > > >     
> > >
> > > Yes, that fits with what I have here, thanks (or where it didn't
> > > quite fit, e.g. ' versus `, I've adjusted to what you have!).  And
> > > thanks for fixing my *orig_pte = *ptep bug, you did point that out
> > > before, but I misunderstood at first.
> > >
> > >  
> > > > Question is what you want me to do now?,
> > > > (Beacuse we are skipping 2.6.31, It is ok to you to tell me something
> > > >  like: "Shut up and let me see what i can get with this madvise" -
> > > >  that from one side.
> > > >  From another side if you want me to do anything please say.
> > > >     
> > >
> > > I had to get a bit further at my end before answering on that,
> > > but now the answer is clear: please do some testing of your RFC
> > > madvise() version (which is what I'm just tidying up a little),
> > > and let me know any bugfixes you find.  Try with SLAB or SLUB or
> > > SLQB debug on e.g. CONFIG_SLUB=y, CONFIG_SLUB_DEBUG=y and boot
> > > option "slub_debug".
> > >   
> >
> > Sure, let me check it.
> > (You do have Andrea patch that fix the "used after free slab entries" ?)
>
> How fast is it crush opps to you?, I compiled it and ran it here on 
> 2.6.30-rc4-mm1 with:
> "Enable SLQB debugging support" and "SLQB debugging on by default, and 
> it run and merge (i am using qemu processes to run virtual machines to 
> merge the pages between them)
>
> ("SLQB debugging on by defaul" mean i dont have to add boot pareameter 
> right?)
>
> Maybe i should try update into newer version of the mm tree? (last 
> commit here is Jul 22)

OK, bug on my side, just got that oppss, will try to fix and send patch.

(Sorry for the noise)

> > > I'm finding, whether with your RFC or my tidyup, that kksmd
> > > soon oopses in get_next_mmlist (or perhaps find_vma): presumably
> > > accessing a vma or mm which already got freed (if you don't have
> > > slab debugging on, it's liable to hang instead).
> > >
> > > (I've also not seen it actually merging yet: if you register
> > > or madvise a large anon area and memset it, the /dev/ksm version
> > > would merge all its pages, but I've not seen the madvise version
> > > do so yet - though maybe there's something stupidly wrong in my
> > > testing, really I'm more worried about the oopses at present.)
> > >
> > > Note that mmotm includes a patch of Nick's which adds a function
> > > madvise_behavior_valid() - you'll need to add your MADVs into its
> > > list to get it to work at all there.
> > >
> > > Here's a patch I added a month or so ago, when trying to experiment
> > > with KSM on all mms: shouldn't be necessary if your mm refcounting
> > > is right, but might help to avoid extra weirdness when things go
> > > wrong: exit_mmap() leaves stale vma pointers around, reckoning
> > > that nobody can be interested by now; but maybe KSM might peep
> > > so better to tidy them up at least while debugging...
> > >
> > > Thanks,
> > > Hugh
> > >
> > > --- old/mm/mmap.c    2009-05-01 13:47:45.000000000 +0100
> > > +++ new/mm/mmap.c    2009-05-03 11:34:47.000000000 +0100
> > > @@ -2112,6 +2112,14 @@ void exit_mmap(struct mm_struct *mm)
> > >      tlb_finish_mmu(tlb, 0, end);
> > >  
> > >      /*
> > > +     * Make sure get_user_pages() and find_vma() etc. will find 
> > > nothing:
> > > +     * this may be necessary for KSM.
> > > +     */
> > > +    mm->mmap = NULL;
> > > +    mm->mmap_cache = NULL;
> > > +    mm->mm_rb = RB_ROOT;
> > > +
> > > +    /*
> > >       * Walk the list again, actually closing and freeing it,
> > >       * with preemption enabled, without holding any MM locks.
> > >       */
> > >   

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