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

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Date:	Mon, 15 Jun 2009 03:05:14 +0300
From:	Izik Eidus <ieidus@...hat.com>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] ksm: write protect pages from inside ksm

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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