Message-ID: <4A358DA7.2080305@redhat.com>
Date:	Mon, 15 Jun 2009 02:54:15 +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:
> 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)

>
>> 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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